myctophids ( myctophidae ) are mesopelagic fishes distributed in all the world s oceans ( dalpadado and gjster 1988 ; cherel et al .
2010 ) , playing an important role in the marine ecosystems ( gjster and kawaguchi 1980 ; cherel et al . 2010 ) as trophic links between zooplankton ( kinzer and schulz 1985 ; shreeve et al . 2009 ) and piscivorous predators ( giske et al . 1990 ; markaida and sosa - nishizaki 2003 ; dokster et al .
they are also likely to play a major role in the vertical flux of organic matter from the upper productive layer to deeper layers ( radchenko 2007 ; hernandez - leon et al .
their vertical distribution and diel vertical migration behavior affect such interactions and processes , making it important to reveal these patterns .
the myctophid glacier lanternfish ( benthosema glaciale ) and the sternoptychidae pearlside ( maurolicus muelleri ) are the dominant mesopelagic fish in norwegian fjords ( aksnes et al .
2004 ; kristoffersen and salvanes 2009 ) . in masfjorden , pearlside dominates acoustic scattering layers ( sl s ) shallower than 200  m , while glacier lanternfish dominates the sl s below 200250  m ( giske et al .
1990 ; bagien et al . 2001 ; kaartvedt et al . 2009 ; dypvik et al .
diel vertical migration ( dvm ) is commonly referred to as a trade - off between feeding opportunities and predation risk induced by changes in light intensity ( clark and levy 1988 ; pearre 2003 ; cohen and forward 2009 ; ringelberg 2010 ) .
thus , light is the proximate cause of dvm influencing the time of migration and vertical extent of migration ( ringelberg and van gool 2003 ; staby and aksnes 2011 ) , while the distribution of food and predation risk are regarded as major drivers for fish vertical distribution and migration ( clark and levy 1988 ; bailey 1989 ; neilson and perry 1990 ; sutton and hopkins 1996 ) .
however , size ( busch and mehner 2012 ) , internal factors such as energy reserves ( hays et al .
2001 ) and hunger ( pearre 2003 ) , and external factors such as currents ( bennett et al . 2002 ) and temperature ( wurtsbaugh and neverman 1988 ; sogard and olla 1996 ; mehner et al .
2010 ) may also influence vertical distribution and migration . in normal dvm ( ndvm )
, individuals ascend toward the surface at night before descending to deeper waters during the day . in the less common inverse dvm ( idvm )
, individuals ascend in the water column at daytime and descend during the night ( pearre 2003 ) .
glacier lanternfish is known to carry out ndvm ( halliday 1970 ; roe and badcock 1984 ; sameoto 1988 ) , idvm ( kaartvedt et al .
2012 ) or display no diel vertical migration ( nodvm ) ( roe and badcock 1984 ; albikovskaya 1988 ; kaartvedt et al .
the depth distribution of myctophids is size dependent with larger fish distributed deeper than smaller individuals ( willis and pearcy 1980 ; gartner et al .
1994 ) , vision ( capabilities ) for detecting prey ( warrant and locket 2004 ) , internal state ( satiation and hunger ) ( cailliet and ebeling 1990 ; staby et al .
2011 ) , and motivation ( rosland and giske 1997 ; busch and mehner 2012 ) are expected to result in a mixture of migration patterns , which may occur simultaneously .
glacier lanternfish feeds on a variety of zooplankton ( gjster 1973 ; roe and badcock 1984 ; sameoto 1988 ) , but seems to prefer calanoid copepods , especially calanus ( sameoto 1988 , 1989 ; balio and aksnes 1993 ; dypvik et al .
2012 ) . the main pattern in seasonal zooplankton abundance and vertical distribution at the site for this study is established from previous studies at the same or adjacent locations . in spring and summer , most zooplankton are distributed in the upper part ( < 30  m ) of the water column ( rasmussen and giske 1994 ) .
however , during autumn and winter , the highest biomass of zooplankton is below 150  m ( giske et al .
this is because zooplankton vanishes from upper layers as primary production declines , but also because of seasonal vertical migration among the main calanoid copepod species , calanus spp .
, which leaves upper waters , descending for  overwintering  in mid - waters ( bagien et al .
myctophids are capable of feeding at mesopelagic depths ( roe and badcock 1984 ; sameoto 1988 ; pusch et al .
2004 ) , so that the seasonal migration of calanus to deep waters may represent an important food source in the daytime habitat of the fish ( dypvik et al .
in norwegian fjords , glacier lanternfish exercise a strong predation pressure on overwintering calanus ( bagien et al .
mesopelagic fish can be studied by use of echo sounders as they tend to aggregate into sl s ( holton 1969 ; god et al .
normally , acoustic studies of mesopelagic fish are carried out in periods restricted by time or seasonality ( collins et al .
2009 ) , and to our knowledge , there is no systematic study addressing how migration patterns in a population of glacier lanternfish , or other myctophids , may vary throughout a year .
however , the use of moored echo sounders can give long - time acoustic data series ( brierley et al .
here we take advantage of the unique opportunity for a long - term study offered by a deep fjord where populations of mesopelagic fish are easily accessible .
we present data from 16  months of continuous acoustic registrations ( july 2007october 2008 ) , enabling us to address the seasonal patterns of diel vertical behavior , unveiling the relative occurrence and consistency of ndvm , idvm , and nodvm by glacier lanternfish . at the outset of this study
, we hypothesized that the patterns of glacier lanternfish dvm would vary seasonally , as recently documented for the pearlside ( staby et al .
given the general seasonal zooplankton dynamics of norwegian fjords , we hypothesize that during spring and summer , when the abundance of potential prey is high in near - surface waters , the glacier lanternfish migrate to the upper part of the water column at night ( ndvm ) to feed , avoiding visual predators in the bright surface waters during daytime .
conversely , we hypothesize that during autumn and winter , when food is sparse in upper waters , and at seasonal high in mid - waters , as seasonally migrating copepods have descended to their overwintering habitat , glacier lanternfish restrain from migrations to the surface at night , rather exploiting the prey in mid - waters by performing idvm toward favored light conditions in shallower waters during daytime ( dypvik et al .
furthermore , for the nodvm layer , we investigate two alternative hypotheses : ( 1 ) the population really does split into migrating and non - migrating components ( pearcy et al .
1979 ) or ( 2 ) migrations are undertaken also among the nodvm component , but not detected by standard methods because they are asynchronous , so that a proportion of the population always is present in deep waters ( sutton and hopkins 1996 ; pearre 2003 ) .
since our approach facilitates studies of individuals , we have the possibility of addressing these competing hypotheses .
the study was undertaken in masfjorden ( ~6052n , ~524e ) , norway ( fig .  1 ) .
the deeper parts of the water column ( ~200390  m ) are particularly in focus as glacier lanternfish dominates the acoustic backscattering , and idvm and nodvm occur in this interval ( dypvik et al .
depicting location of the bottom - mounted echo sounder ( gray circle ) at 392  m depth in masfjorden , norway map depicting location of the bottom - mounted echo sounder ( gray circle ) at 392  m depth in masfjorden , norway a mooring with an upward - facing 38  khz simrad ek60 split - beam echo sounder ( 7.1 beam width ) , cabled to shore for power and transmission of data , was deployed at 392  m in july 2007 and retrieved in october 2008 .
detailed descriptions of the echo sounder system and parameter settings are given in kaartvedt et al .
data were recorded continuously , apart for intermittent periods following power failures , giving a total of 218 full days of records spanning all seasons .
a minimum number of 4  days per month were included in this study ( i.e. sept . 2007 and
we classify september  november as autumn , december  february as winter , march  may as spring , and june  august as summer .
the results are presented as average echograms illustrating the average sv ( db ) values for the entire water column ( ~0390  m ) for each month , and in greater detail from selected months representing each season for the section ~200390  m , which is of particular interest for this study ( called monthly echograms ) .
sv values are logarithmic measurements of the volume acoustic backscattering or accumulated ts ( target strength ) , ts representing the echo of a single individual ( maclennan and simmonds 1992 ) .
the seasonal patterns of the vertical distribution and migration of glacier lanternfish is assessed by studying the scattering layers in monthly echograms .
these monthly echograms were made in matlab by dividing each day into 30  s intervals , and for each time interval average all the pings for the given month . in this way
, all the acoustic records from the entire registration period are included in the presentations .
in addition , to better illustrate any activity in the nodvm layer , which by definition was expected to have little vertical migration , we randomly picked 1  day for each season ( 18 october 2007 ; 2 february ; 12 march 2008 ; 8 august 2008 ) , focusing on the 300390  m depth interval . for these days , we furthermore selectively chose examples where vertical swimming was seen in the compressed , daily echograms , and portrayed these at higher resolution for better depicting individual behavior .
abundance estimates to identify the variation in concentration of glacier lanternfish in different depth intervals between day and night were made by echo integration .
this was done with day and depth divided into blocks of 1  h and 10  m respectively between ~250 and 390  m for days where 24  h of echo data were successfully retrieved ( from min .
time for sunrise and sunset was set to the 15th of each corresponding month , as an approximate for each month , determining time allocated to day and night .
glacier lanternfish makes up the main part of the volume backscattering ( sv ) at 90  db , while larger fish results in sv values greater than 64  db ( bagien et al . 2001 ) .
thus , values retrieved by echo integration at 64  db were subtracted from the echo integration at 90  db to exclude fish larger than glacier lanternfish ( bagien et al .
the concentrations ( individuals m ) were derived from measurements of sv and target strength ( ts , see description below ) by dividing the linear sv values with the linear ts ( maclennan and simmonds 1992 ) .
ts depends on the size , anatomy , and behavior of the organism , as well as the frequency of the echo sounder ( maclennan and simmonds 1992 ) . for precise estimation of concentration
, the ts of glacier lanternfish was obtained monthly by means of automatic target tracking ( tt ) , performing tt for 3  days of every month within a range of 1050  m from the transducer ( ~340380  m depth ) .
this limited range was chosen as the resolution of acoustic data decreases with distance from the transducer , and so does the precision of the ts estimates .
minimum track length was set at 10 ping , maximum ping gap to 1 ping and gating range to 0.3  m during tracking .
ts values between 65 and 50  db were regarded as glacier lanternfish , as ts values stronger than 50  db indicate fish of greater size ( foote 1980 ; maclennan and simmonds 1992 ) .
a minimum value of 65  db was chosen in accordance with a previous acoustic study of glacier lanternfish ( torgersen and kaartvedt 2001 ) .
tracks of individual fish obtained by tt were also used for analysis of swimming behavior .
vertical swimming speeds of individuals were analyzed together with visual inspection of daily 24  h echograms in order to investigate any asynchronicity in the dvm pattern of the deeper living part of the population .
both tt and echo integration were performed in the post processing program sonar_5 pro version 5.9.9 ( balk and lindem 2007 ) .
the migration patterns below 250  m as assessed from the monthly echograms were compared with changes in concentration as assessed based on combining the measurements of sv and ts .
the maximum concentration of glacier lanternfish estimated below 250  m each day was assumed to reflect the total population below 250  m. increase and/or decrease in concentration , before and after the time of migration of a specific layer of glacier lanternfish , was used as a proxy for the relative proportion of glacier lanternfish with nodvm , idvm , and ndvm .
the speed of migration by individuals in sl s was calculated by analyzing the distance of descent / ascent over time in sl s .
the continuous acoustic measurements were complemented by sampling during research cruises 14 november 2007 with r / v  haakon mosby 
( university of bergen , imr ) and 37 october 2008 with r / v  trygve braarud  ( university of oslo ) .
results from the physical oceanography , zooplankton , and trawling studies from these field campaigns are used in the interpretations , but are presented elsewhere ( staby et al .
the behavioral patterns described in this study can with confidence be ascribed to glacier lanternfish .
previous studies covering spring and summer ( rasmussen and giske 1994 ; kaartvedt et al .
unpublished ) , autumn ( bagien et al . 2001 ; kaartvedt et al . 2009 ) , winter ( giske et al . 1990 ; balio and aksnes 1993 ; bagien et al . 2001 ) , and trawl data obtained during the present study ( staby et al . 2011 ; dypvik et al . 2012 ) , as well as subsequent studies ( kaartvedt et al .
unpublished ) have shown that glacier lanternfish are the main cause of backscattering deeper than 250  m in masfjorden .
catches at these depths consist of specimens > 4.5  cm , with a prevalence of individuals > 6  cm below 300  m ( kaartvedt et al . 2009 ; dypvik et al . 2012 ) , that is adult individuals ( gjster 1981 ) .
pelagic shrimps are also common in the deep waters of masfjorden ( kaartvedt et al . 1988 , 2009 ) and may contribute somewhat to the backscatter .
however , the sl s below 200  m in masfjorden are pronounced also at 18  khz ( kaartvedt et al . 2008 ) , signifying that the backscatter mainly consists of mesopelagic fish ( torgersen and kaartvedt 2001 ; love et al . 2004 ; god et al . 2009 ; kloser et al . 2009 ) .
furthermore , the average ts of the targets studied here ( see  results  ;  target strength and concentration estimates in deep water  ) is in accordance with previous studies of lanternfishes ( torgersen and kaartvedt 2001 ; yasuma et al .
2009 ) , while shrimps have a ts about one magnitude weaker than that of glacier lanternfish ( benoit - bird and au 2001 ; torgersen and kaartvedt 2001 ) .
in summer , there were four sl s performing ndvm to the surface at night , and one deep layer with no apparent diel behavior ( fig .  2 ) .
the upper of these layers ( ~70200  m ) were ascribed to pearlside as they are found to dominate at these depths ( the seasonal dvm patterns of pearlside are discussed in staby et al .
the two deeper layers ( > 250  m during daytime ) were ascribed to glacier lanternfish with ndvm and nodvm , respectively . zooming in on the deeper parts of the water column
illustrates more clearly how organisms carrying out ndvm in summer descended to waters below ~250  m at daytime , and ascended at night ( fig . 
2monthly average echograms from the 38-khz echo sounder covering the water column ( ~0390  m ) for 24  h in july 2007october 2008 .
the dominant signal ( green ) and backscattering at shallower depth ( < 200  m ) is dominated by pearlside , while glacier lanternfish dominate below ~250 
the boxes indicate the intervals where the concentration ( individuals m ) of glacier lanternfish is calculated ( see fig . 
the color scale refers to average volume backscattering ( s 
 v ) for each month ; dark red illustrates the strongest , and white the weakest backscatter .
the s 
 v - thresholds are set to 50 and 85  db for the best possible illustration of the sl s .
black and white 
 bars above echograms depict night and day separated by times for sunrise and sunset for the 15th each month .
3monthly average echograms from the 38-khz echo sounder ( covering 24  h ) illustrating the deeper part of the water column ( ~200390  m ) in summer ( represented by august 2008 ) , autumn ( represented by september 2007 ) , winter ( represented by january 2008 ) , and spring ( represented by may 2008 ) .
arrows in echograms indicate the different behavioral patterns ; no diel vertical migration ( nodvm ) , inverse diel vertical migration ( idvm ) , and normal diel vertical migration ( ndvm ) . the color scale refers to average volume backscattering ( s 
 v ) for each month ; dark red illustrates the strongest , and white the weakest backscatter .
black and white bars above echograms depict night and day separated by times for sunrise and sunset for the 15th each month .
time is given as utc ( local standard time 1  h ) monthly average echograms from the 38-khz echo sounder covering the water column ( ~0390  m ) for 24  h in july 2007october 2008 .
the dominant signal ( green ) and backscattering at shallower depth ( < 200  m ) is dominated by pearlside , while glacier lanternfish dominate below ~250 
the boxes indicate the intervals where the concentration ( individuals m ) of glacier lanternfish is calculated ( see fig . 
the color scale refers to average volume backscattering ( s 
 v ) for each month ; dark red illustrates the strongest , and white the weakest backscatter .
the s 
 v - thresholds are set to 50 and 85  db for the best possible illustration of the sl s .
black and white 
 bars above echograms depict night and day separated by times for sunrise and sunset for the 15th each month .
time is given as utc ( local standard time 1  h ) monthly average echograms from the 38-khz echo sounder ( covering 24 
h ) illustrating the deeper part of the water column ( ~200390  m ) in summer ( represented by august 2008 ) , autumn ( represented by september 2007 ) , winter ( represented by january 2008 ) , and spring ( represented by may 2008 ) .
arrows in echograms indicate the different behavioral patterns ; no diel vertical migration ( nodvm ) , inverse diel vertical migration ( idvm ) , and normal diel vertical migration ( ndvm ) .
the color scale refers to average volume backscattering ( s 
 v ) for each month ; dark red illustrates the strongest , and white the weakest backscatter .
black and white bars above echograms depict night and day separated by times for sunrise and sunset for the 15th each month .
time is given as utc ( local standard time 1  h ) in autumn , ndvm was still present above ~200  m , but was less noticeable below ~200  m ( fig .  2 ) .
idvm of acoustic targets ascribed to glacier lanternfish was now evident below the pearlside sl ( figs .  2 , 3 ) .
an apparent non - migrating layer of glacier lanternfish occurred from 300  m to the bottom ( figs .  2 , 3 ) .
in winter , there were strong echoes of vertical migrating juvenile pearlside , and largely non - migrating adult pearlside in the upper 200  m ( fig .  2 ) .
the sl of individuals with idvm ( glacier lanternfish ) was still apparent , although less pronounced than in autumn ( figs .  2 , 3 ) .
the deepest non - migrating layer was now at its seasonal high in level of backscatter ( fig .  2 ) .
in spring , several sl s with ndvm appeared between ~100210  m in daytime ( march and april ) and in the upper ~100  m at night ( fig .  2 ) .
, individuals performing ndvm descended to ~300  m in daytime , indicating a large portion of the glacier lanternfish population with this behavior ( figs .  2 , 3 ) .
m ( glacier lanternfish ) , yet with a seasonal low in backscatter ( fig .  2 ) . estimated proportions with different dvm modes were calculated for each month based on the echograms and the echo integration from 250 to 390  m ( fig . 
ndvm was performed by ~2555  % of these fish in summer , while this behavior ceased in autumn and winter ( fig . 
however , individuals performing ndvm re - appeared in early spring , and the proportion performing ndvm increased toward late spring ( from ~13  % in march to ~51  % in may ) ( fig .  4 ) .
the proportion performing idvm increased from late summer ( ~15  % in august ) to the beginning of autumn ( 70  % in september 2007 ) , then decreased throughout the winter ( 24  % in february 2008 ) . in spring and the first part of summer , idvm was not recorded ( fig . 
nodvm was suggested for ~4575  % of the population at these depths during spring and summer , but this proportion decreased from summer to autumn ( ~3040  % in september 2007 and 2008 ) , before it increased toward the winter months ( fig . 
4the relative frequency of the population of glacier lanternfish ( found below 250  m during daytime ) with different dvm behaviors from july 2007october 2008 .
no diel vertical migration ( nodvm ) , inverse diel vertical migration ( idvm ) , and normal diel vertical migration ( ndvm ) are illustrated by black , white , and gray colors , respectively the relative frequency of the population of glacier lanternfish ( found below 250  m during daytime ) with different dvm behaviors from july 2007october 2008 .
no diel vertical migration ( nodvm ) , inverse diel vertical migration ( idvm ) , and normal diel vertical migration ( ndvm ) are illustrated by black , white , and gray colors , respectively visual inspection of 24  h echograms suggested little systematic vertical swimming among the nodvm component in relation to time of day , as exemplified by 1  day each season in fig . 
fish at these depths commonly appeared to drift up and down in a consistent pattern suggesting internal wave motions ( fig . 
the limited vertical movement was reflected in the results from the year - round tt . of more than 200,000 tracks retrieved by tt
however , intermittent vertical swimming was recorded in which individuals ascended or descended in a step - wise pattern ( fig . 
the maximum vertical swimming speed detected was ~18.4  cm / s . in comparison ,
the approximately ascending and descending speeds of the migrating layers were 34  cm / s for the ndvm layer and 0.50.8  cm / s for the idvm layer .
in addition to such marked vertical relocations , seemingly passively drifting fishes once in a while slightly adjusted their vertical distribution , subsequently taking up their apparent torpid behavior.fig .  5 
 a 
d 24-h echograms from the bottom - mounted 38  khz echo sounder on 8 august 2008 ( a ) , 18 october 2007 ( b ) , 2 february ( c ) , and 12 march 2008 ( d ) .
the echograms are zoomed in at ~300390  m. the outlined boxes are presented in higher resolution in ( e  h ) .
the color scale refers to average volume backscattering ( s 
 v ) ; dark red illustrates the strongest , and white the weakest backscatter .
black and white bars above echograms depict night and day separated by times for sunrise and sunset .
time is given as utc ( local standard time 1  h ) 
 a 
d 24-h echograms from the bottom - mounted 38  khz echo sounder on 8 august 2008 ( a ) , 18 october 2007 ( b ) , 2 february ( c ) , and 12 march 2008 ( d ) . the echograms are zoomed in at ~300390  m. the outlined boxes are presented in higher resolution in ( e  h ) .
the color scale refers to average volume backscattering ( s 
 v ) ; dark red illustrates the strongest , and white the weakest backscatter .
black and white bars above echograms depict night and day separated by times for sunrise and sunset .
time is given as utc ( local standard time 1  h ) the average ts ( db ) of glacier lanternfish as measured between ~340 and 380  m were always in the range of approximately 57.5 to 59.5  db .
within this limited range , the ts distribution of glacier lanternfish showed two peaks ( in both summers ) and decreased from the autumn to spring ( fig . 
6average ts ( db ) of glacier lanternfish ( total n  =  209,768 ) calculated from ~340 to 380  m for each month throughout the study period ( july 2007october 2008 ) average ts ( db ) of glacier lanternfish ( total n  =  209,768 ) calculated from ~340 to 380  m for each month throughout the study period ( july 2007october 2008 ) the ts values were used in assessing the numerical densities of glacier lanternfish .
the concentration of fish was usually in the range of 0.0050.015 individuals m ( fig . 
however , the maximum density , just below 0.02 individuals m , was recorded in july 2008 in the deepest interval ( 300390  m ) , and in october 2008 in the shallower interval ( 250300  m ) ( fig .  7a , b).fig . 
7 
 a , b estimated monthly average concentration ( individuals per m ) of glacier lanternfish during day and night from ( a ) 250300  m and ( b ) 300390  m during july 2007october 2008 .
the white and black squares indicate the median daytime and nighttime concentration estimates , respectively .
the black lines illustrate the 1st and 3rd quartile , while the x indicates the average estimated concentration 
 a , b estimated monthly average concentration ( individuals per m ) of glacier lanternfish during day and night from ( a ) 250300  m and ( b ) 300390  m during july 2007october 2008 .
the white and black squares indicate the median daytime and nighttime concentration estimates , respectively .
the black lines illustrate the 1st and 3rd quartile , while the x indicates the average estimated concentration in spring and summer , when ndvm is the dominating mode of migration , the daytime concentration of the interval ~250300  m was higher than during night ( fig . 
below 300  m , where the nodvm mode were dominating , relatively little concentration fluctuations between day and night occurred , except for june  august 2008 ( fig . 
7b ) when the ndvm layers entered those depths at daytime ( fig .  2 ) . in the months where idvm were the dominating mode of migration ( i.e. september  february ) , the concentration of glacier lanternfish decreased during daytime in the interval ~250300  m ( fig . 
7a ) , as the inverse vertical migrators that were distributed from ~220 to 300  m during nighttime ascended to mid - waters ( in the lower part of the strong sl between ~150220  m ) during the day ( figs .  2 , 3 ) .
in summer , there were four sl s performing ndvm to the surface at night , and one deep layer with no apparent diel behavior ( fig .  2 ) .
the upper of these layers ( ~70200  m ) were ascribed to pearlside as they are found to dominate at these depths ( the seasonal dvm patterns of pearlside are discussed in staby et al .
the two deeper layers ( > 250  m during daytime ) were ascribed to glacier lanternfish with ndvm and nodvm , respectively . zooming in on the deeper parts of the water column
illustrates more clearly how organisms carrying out ndvm in summer descended to waters below ~250  m at daytime , and ascended at night ( fig . 
2monthly average echograms from the 38-khz echo sounder covering the water column ( ~0390  m ) for 24  h in july 2007october 2008 .
the dominant signal ( green ) and backscattering at shallower depth ( < 200  m ) is dominated by pearlside , while glacier lanternfish dominate below ~250 
the boxes indicate the intervals where the concentration ( individuals m ) of glacier lanternfish is calculated ( see fig . 
the color scale refers to average volume backscattering ( s 
 v ) for each month ; dark red illustrates the strongest , and white the weakest backscatter .
the s 
 v - thresholds are set to 50 and 85  db for the best possible illustration of the sl s .
black and white 
 bars above echograms depict night and day separated by times for sunrise and sunset for the 15th each month .
3monthly average echograms from the 38-khz echo sounder ( covering 24  h ) illustrating the deeper part of the water column ( ~200390  m ) in summer ( represented by august 2008 ) , autumn ( represented by september 2007 ) , winter ( represented by january 2008 ) , and spring ( represented by may 2008 ) .
arrows in echograms indicate the different behavioral patterns ; no diel vertical migration ( nodvm ) , inverse diel vertical migration ( idvm ) , and normal diel vertical migration ( ndvm ) . the color scale refers to average volume backscattering ( s 
 v ) for each month ; dark red illustrates the strongest , and white the weakest backscatter .
black and white bars above echograms depict night and day separated by times for sunrise and sunset for the 15th each month .
time is given as utc ( local standard time 1  h ) monthly average echograms from the 38-khz echo sounder covering the water column ( ~0390  m ) for 24  h in july 2007october 2008 .
the dominant signal ( green ) and backscattering at shallower depth ( < 200  m ) is dominated by pearlside , while glacier lanternfish dominate below ~250 
the boxes indicate the intervals where the concentration ( individuals m ) of glacier lanternfish is calculated ( see fig . 
the color scale refers to average volume backscattering ( s 
 v ) for each month ; dark red illustrates the strongest , and white the weakest backscatter .
the s 
 v - thresholds are set to 50 and 85  db for the best possible illustration of the sl s .
black and white 
 bars above echograms depict night and day separated by times for sunrise and sunset for the 15th each month .
time is given as utc ( local standard time 1  h ) monthly average echograms from the 38-khz echo sounder ( covering 24 
h ) illustrating the deeper part of the water column ( ~200390  m ) in summer ( represented by august 2008 ) , autumn ( represented by september 2007 ) , winter ( represented by january 2008 ) , and spring ( represented by may 2008 ) .
arrows in echograms indicate the different behavioral patterns ; no diel vertical migration ( nodvm ) , inverse diel vertical migration ( idvm ) , and normal diel vertical migration ( ndvm ) .
the color scale refers to average volume backscattering ( s 
 v ) for each month ; dark red illustrates the strongest , and white the weakest backscatter .
black and white bars above echograms depict night and day separated by times for sunrise and sunset for the 15th each month .
time is given as utc ( local standard time 1  h ) in autumn , ndvm was still present above ~200  m , but was less noticeable below ~200  m ( fig .  2 ) .
idvm of acoustic targets ascribed to glacier lanternfish was now evident below the pearlside sl ( figs .  2 , 3 ) .
an apparent non - migrating layer of glacier lanternfish occurred from 300  m to the bottom ( figs .  2 , 3 ) .
in winter , there were strong echoes of vertical migrating juvenile pearlside , and largely non - migrating adult pearlside in the upper 200  m ( fig .  2 ) .
the sl of individuals with idvm ( glacier lanternfish ) was still apparent , although less pronounced than in autumn ( figs .  2 , 3 ) .
the deepest non - migrating layer was now at its seasonal high in level of backscatter ( fig .  2 ) .
in spring , several sl s with ndvm appeared between ~100210  m in daytime ( march and april ) and in the upper ~100  m at night ( fig .  2 ) .
, individuals performing ndvm descended to ~300  m in daytime , indicating a large portion of the glacier lanternfish population with this behavior ( figs .  2 , 3 ) .
m ( glacier lanternfish ) , yet with a seasonal low in backscatter ( fig .  2 ) .
estimated proportions with different dvm modes were calculated for each month based on the echograms and the echo integration from 250 to 390  m ( fig .  4 ) .
ndvm was performed by ~2555  % of these fish in summer , while this behavior ceased in autumn and winter ( fig . 
however , individuals performing ndvm re - appeared in early spring , and the proportion performing ndvm increased toward late spring ( from ~13  % in march to ~51  % in may ) ( fig . 
the proportion performing idvm increased from late summer ( ~15  % in august ) to the beginning of autumn ( 70  % in september 2007 ) , then decreased throughout the winter ( 24  % in february 2008 ) . in spring and the first part of summer ,
nodvm was suggested for ~4575  % of the population at these depths during spring and summer , but this proportion decreased from summer to autumn ( ~3040  % in september 2007 and 2008 ) , before it increased toward the winter months ( fig . 
.  4the relative frequency of the population of glacier lanternfish ( found below 250  m during daytime ) with different dvm behaviors from july 2007october 2008 .
no diel vertical migration ( nodvm ) , inverse diel vertical migration ( idvm ) , and normal diel vertical migration ( ndvm ) are illustrated by black , white , and gray colors , respectively the relative frequency of the population of glacier lanternfish ( found below 250  m during daytime ) with different dvm behaviors from july 2007october 2008 .
no diel vertical migration ( nodvm ) , inverse diel vertical migration ( idvm ) , and normal diel vertical migration ( ndvm ) are illustrated by black , white , and gray colors , respectively
visual inspection of 24  h echograms suggested little systematic vertical swimming among the nodvm component in relation to time of day , as exemplified by 1  day each season in fig . 
fish at these depths commonly appeared to drift up and down in a consistent pattern suggesting internal wave motions ( fig . 
the limited vertical movement was reflected in the results from the year - round tt . of more than 200,000 tracks retrieved by tt , ~85  % revealed vertical relocation speed < 0.4  cm / s . however ,
intermittent vertical swimming was recorded in which individuals ascended or descended in a step - wise pattern ( fig . 
5e  h ) . the maximum vertical swimming speed detected was ~18.4  cm / s . in comparison ,
the approximately ascending and descending speeds of the migrating layers were 34  cm / s for the ndvm layer and 0.50.8  cm / s for the idvm layer .
in addition to such marked vertical relocations , seemingly passively drifting fishes once in a while slightly adjusted their vertical distribution , subsequently taking up their apparent torpid behavior.fig .  5 
 a  d 24-h echograms from the bottom - mounted 38  khz echo sounder on 8 august 2008 ( a ) , 18 october 2007 ( b ) , 2 february ( c ) , and 12 march 2008 ( d ) .
the echograms are zoomed in at ~300390  m. the outlined boxes are presented in higher resolution in ( e  h ) . the color scale refers to average volume backscattering ( s 
 v ) ; dark red illustrates the strongest , and white the weakest backscatter .
black and white bars above echograms depict night and day separated by times for sunrise and sunset .
time is given as utc ( local standard time 1  h ) 
 a 
d 24-h echograms from the bottom - mounted 38  khz echo sounder on 8 august 2008 ( a ) , 18 october 2007 ( b ) , 2 february ( c ) , and 12 march 2008 ( d ) .
the echograms are zoomed in at ~300390  m. the outlined boxes are presented in higher resolution in ( e  h ) .
the color scale refers to average volume backscattering ( s 
 v ) ; dark red illustrates the strongest , and white the weakest backscatter .
black and white bars above echograms depict night and day separated by times for sunrise and sunset .
the average ts ( db ) of glacier lanternfish as measured between ~340 and 380  m were always in the range of approximately 57.5 to 59.5  db . within this limited range ,
the ts distribution of glacier lanternfish showed two peaks ( in both summers ) and decreased from the autumn to spring ( fig . 
6average ts ( db ) of glacier lanternfish ( total n  =  209,768 ) calculated from ~340 to 380  m for each month throughout the study period ( july 2007october 2008 ) average ts ( db ) of glacier lanternfish ( total n  =  209,768 ) calculated from ~340 to 380  m for each month throughout the study period ( july 2007october 2008 ) the ts values were used in assessing the numerical densities of glacier lanternfish .
the concentration of fish was usually in the range of 0.0050.015 individuals m ( fig . 
however , the maximum density , just below 0.02 individuals m , was recorded in july 2008 in the deepest interval ( 300390  m ) , and in october 2008 in the shallower interval ( 250300  m ) ( fig . 
7 
 a , b estimated monthly average concentration ( individuals per m ) of glacier lanternfish during day and night from ( a ) 250300  m and ( b ) 300390  m during july 2007october 2008 .
the white and black squares indicate the median daytime and nighttime concentration estimates , respectively .
the black lines illustrate the 1st and 3rd quartile , while the x indicates the average estimated concentration 
 a , b estimated monthly average concentration ( individuals per m ) of glacier lanternfish during day and night from ( a ) 250300  m and ( b ) 300390  m during july 2007october 2008 .
the white and black squares indicate the median daytime and nighttime concentration estimates , respectively .
the black lines illustrate the 1st and 3rd quartile , while the x indicates the average estimated concentration in spring and summer , when ndvm is the dominating mode of migration , the daytime concentration of the interval ~250300  m was higher than during night ( fig . 
below 300  m , where the nodvm mode were dominating , relatively little concentration fluctuations between day and night occurred , except for june  august 2008 ( fig . 
7b ) when the ndvm layers entered those depths at daytime ( fig .  2 ) . in the months where idvm were the dominating mode of migration ( i.e. september  february ) , the concentration of glacier lanternfish decreased during daytime in the interval ~250300  m ( fig . 
7a ) , as the inverse vertical migrators that were distributed from ~220 to 300  m during nighttime ascended to mid - waters ( in the lower part of the strong sl between ~150220  m ) during the day ( figs .  2 , 3 )
the 16-month acoustic data series revealed three different diel behavioral modes of glacier lanternfish . in ndvm , individuals ascend toward the surface at night and descend back to deeper waters in the morning . in idvm
, individuals ascend toward mid - waters at daytime , before descending at night . in nodvm
a clear seasonal pattern , with ndvm prevailing during spring and summer , and idvm prevailing during autumn and winter , was also apparent .
all through the year , nodvm occurred in the water column deeper than ~300  m. this diversity in diel behavior of glacier lanternfish has previously been observed during autumn in masfjorden by kaartvedt et al .
note , however , that while the acoustic results unveil the main patterns in diel behavior for the part of the population below ~200  m , they do not provide the entire picture for the whole population .
only idvm was recorded acoustically in october and november , yet trawl catches documented some ndvm ( kaartvedt et al .
also previous studies have documented ndvm of glacier lanternfish in fall and winter ( kaartvedt et al .
1988 ; giske et al . 1990 ; bagien et al . 2001 ) . some shallower - living glacier lanternfish intermingle with , and
thus become inseparable in the acoustic signatures of the sl dominated by adult pearlside ( torgersen and kaartvedt 2001 ) .
shallower - living glacier lanternfish are mostly smaller ( dypvik et al . 2012 ) and might be too small to be properly detected at 38  khz .
the largely consistency in ts values in deep water underlines that we likely have addressed the adult population throughout this study period .
the slight apparent seasonal variation in ts does not seem to correlate with behavioral changes . during spring and summer ,
, the concentration of zooplankton peaks in the surface layers , and significantly lowers in the deeper waters ( atkinson and peck 1988 ; rasmussen and giske 1994 ; richter 1994 ) .
ndvm among lanternfishes is usually related to avoidance of visual predators in bright surface waters at daytime and subsequent feeding on zooplankton in the surface layers at night ( holton 1969 ; dalpadado and gjster 1987 ; kinzer et al . 1993 ) .
the pay - off from ndvm will decrease as surface plankton becomes less abundant throughout fall and winter , while at the same time , seasonally migrating copepods return to depth ( speirs et al . 2006 ; broms and melle 2007 ) .
. however , small individuals with less energy reserves , larger motivation , or less capability for feeding in deep water may continue to migrate toward the surface at night ( giske and aksnes 1992 ) , while larger fish may opt for other strategies such as staying in deeper waters feeding on overwintering calanoid copepods . in accordance with this ,
mainly 2-year or older glacier lanternfish are distributed deeper than ~250  m , while smaller individuals are distributed in shallower waters , during autumn ( dypvik et al .
similar size - dependent depth distribution has also been observed in other studies of lanternfishes ( willis and pearcy 1980 ; gartner et al .
idvm became apparent in august and was recorded until march . in winter , the bulk of zooplankton biomass is distributed in mid - waters ( giske et al .
( 2012 ) showed that inversely migrating glacier lanternfish were foraging on overwintering calanus in mid - waters during the day .
other studies have shown that calanus is an important part of the glacier lanternfish diet during fall and winter ( gjster 1973 ; balio and aksnes 1993 ) . the seasonal occurrence of idvm seems to harmonize well with the overwintering period for calanus finmarchicus ( bagien et al . 2001 ; speirs et al .
2006 ) , in both the timing of initiation of this behavior , and its termination in spring ( heath 1999 ; astthorsson and gislason 2003 ; speirs et al . 2006 ; broms and melle 2007 ) . by staying in deeper waters performing idvm , glacier
lanternfish may optimize their energy budget and reduce the predation risk ( dypvik et al .
in addition , light decreases rapidly with depth in seawater ( srnes and aksnes 2006 ; aksnes et al . 2009 ) . a lanternfish swimming toward shallower waters , while still staying below ~200  m , will thus experience an increase of light in the surroundings , which would favor visual feeding by the low - light adapted lanternfish ( warrant and locket 2004 ) .
thus , it seems likely that individuals with idvm ascend toward the overwintering layer of calanus at daytime to feed at favored light levels , sinking back to deeper layers at night .
however , the proportion of the population with idvm decreases through the winter , which may reflect the reduced concentration of calanus from autumn through winter ( bagien et al .
individuals of glacier lanternfish with nodvm occurred all year at depths greater than ~300  m. dypvik
et al . ( 2012 ) found that the individuals with nodvm during autumn were the largest individuals of the glacier lanternfish population .
individuals of greatest size tend to have larger eyes and might be better adapted than smaller individuals to detect prey in low light ( warrant and locket 2004 ) .
they are also better adapted for handling large prey such as krill and shrimps , which occur deep in the water column ( kaartvedt et al .
2012 ) , and which represent a much higher dividend than copepods due to their much larger size ( falk - petersen 1981 ; tande 1982 ) .
we therefore suggest that individuals with nodvm are able to feed in the restricted light levels of the deep waters .
the behavior of individuals in deep water ( > 300  m ) suggested that the nodvm group indeed largely consisted of non - migrating individuals , and that consistent backscatter in deep water is not a result of asynchronous migrations , but that the population rather splits into migrating and non - migrating individuals , as initially suggested by pearcy et al .
( 1979 ) . nevertheless , single individuals were observed swimming upwards or downwards in a step - wise fashion , as also observed by kaartvedt et al .
intermittent vertical relocations , ascent / descent in a step - wise fashion , would help the fish scan different parts of the water column for prey .
fish may also use their lateral lines for tactile prey detection in darkness ( janssen et al .
1999 ) , and such behavior would be in accordance with both visual ( obrien et al .
5b ) suggests a prominence of ascent at day , so we can not reject the possibility that some of these deep - living individuals took part in idvm at that time . yet
, most step - wise movements seemed to represent infrequent re - locations within the layer .
a wide variety of lanternfishes are capable of feeding at depths greater than several hundreds of meters ( pearcy et al .
1979 ; roe and badcock 1984 ; sameoto 1988 ) , and nodvm has previously been observed among several species of lanternfish ( pearcy et al .
however , different species of calanus occur in different geographic regions and , as well as other genera , perform seasonal vertical migration , overwintering in deeper water ( atkinson and peck 1988 ; richter 1995 ; peterson 1998 ; broms and melle 2007 ) .
we therefore hypothesize that idvm and nodvm are widespread behaviors in areas where concentrations of potential prey are distributed in deeper waters , and that similar plasticity in dvm behavior as observed for glacier lanternfish in masfjorden might occur in other populations of lanternfishes in areas with seasonally migrating zooplankton , such as overwintering calanoid copepods .
long - term acoustic measurements might be used to reveal such dvm plasticity in other areas , information that could be used to gain knowledge about interactions between lanternfishes , their prey and predators , as well as lanternfishes contribution to the vertical flux of organic matter .
the vertical distribution and diel vertical migration of lanternfishes can affect such interactions and processes , making it imperative to further reveal these patterns .
 the seasonal variations in glacier lanternfish ( benthosema glaciale ) vertical distribution and diel vertical migration ( dvm ) were studied by use of a bottom - mounted upward - facing 38  khz echo sounder deployed at 392  m depth and cabled to shore in masfjorden ( ~6052n , ~524e ) , norway . 
 acoustic data from july 2007october 2008 were analyzed , and scattering layers below ~220  m during daytime were attributed to glacier lanternfish based on net sampling in this , and previous studies , as well as from analysis of the acoustic data . at these depths , 
 three different diel behavioral strategies were apparent : normal diel vertical migration ( ndvm ) , inverse dvm ( idvm ) , and no dvm ( nodvm ) . 
 nodvm was present all year , while idvm was present in autumn and winter , and ndvm was present during spring and summer . 
 the seasonal differences in dvm behavior seem to correlate with previously established seasonal distribution of prey . 
 we hypothesize that in regions with seasonally migrating zooplankton , such as where calanoid copepods overwinter at depth , similar plasticity in dvm behavior might occur in other populations of lanternfishes . 
Introduction
Materials and methods
Results
Main patterns in acoustic backscatter
Relative proportion of migration patterns
Individual behavior of glacier lanternfish
Target strength and concentration estimates in deep water
Discussion