because hypoxia can make deeper cooler waters unavailable in the summer or overlaps with nursery habitat the combined effects of temperature and oxygen can result in habitat compression or squeeze that affects both pelagic and demersal species coutant niklitschek and secor pearce and balcom the location and seasonal persistence of hypoxia appears to be particularly damaging to anadromous and catadromous species that require use of and transit through both saline and freshwater portions of systems to complete life cycles breitburg etal large stretches of river estuaries such as the mersey thames elbe and delaware have been described as devoid of fish prior to implementation of primary sewage treatment thiel etal