Editing Atlantic cod (section)

==Behaviour==

[[File:Atlantic cod under a shipwreck.jpg|thumb|left|Atlantic cod are [[demersal fish]]—they prefer sea bottoms with coarse sediments.<ref>[http://www.fishwatch.gov/seafood_profiles/species/cod/species_pages/atlantic_cod.htm Atlantic cod] ''NOAA FishWatch''. Retrieved 5 November 2012.</ref>]][[File:Jonge kabeljauwen oftewel gul rondom een wrak met andere vissen-4885742.webm|left|upright=1.35|thumb|Young Atlantic cod avoid larger cod and pouting (''[[Trisopterus luscus]]'') and crabs on a wreck in the southern [[North Sea]]]]

===Shoaling===
[[File:Kabeljauwen houden zich op bij een wrak en bevrijden zichzelf-4885741.webm|upright=1.35|thumb|[[Shoaling and schooling|Shoaling]] Atlantic cod on a wreck in the [[North Sea]]]]

Atlantic cod are a [[Shoaling and schooling|shoaling]] species and move in large, size-structured aggregations. Larger fish act as scouts and lead the shoal's direction, particularly during post [[Spawn (biology)|spawning]] [[Fish migration|migrations]] inshore for feeding. Cod actively feed during migration and changes in shoal structure occur when food is encountered. Shoals are generally thought to be relatively leaderless, with all fish having equal status and an equal distribution of resources and benefits.<ref name=Pitcher>{{cite book|last=Pitcher|first=TJ, Parrish JK|title=Functions of shoaling behaviour in teleosts|year=1993|publisher=Chapman and Hall|pages=363–427}}</ref>  However, some studies suggest that leading fish gain certain feeding benefits. One study of a migrating Atlantic cod shoal showed significant variability in feeding habits based on size and position in the shoal. Larger scouts consumed a more variable, higher quantity of food, while trailing fish had less variable diets and consumed less food. Fish distribution throughout the shoal seems to be dictated by fish size, and ultimately, the smaller lagging fish likely benefit from shoaling because they are more successful in feeding in the shoal than they would be if migrating individually, due to social facilitation.<ref name=DeBlois>{{cite journal|last=DeBlois|first=Elisabeth M.|author2=Rose, George A.|title=Cross-shoal variability in the feeding habits of migrating Atlantic cod (Gadus morhua)|journal=Oecologia|date=1 January 1996|volume=108|issue=1|pages=192–196|doi=10.1007/BF00333231|pmid=28307750|bibcode=1996Oecol.108..192D|s2cid=1592090}}</ref>

===Predation===
Atlantic cod are apex predators in the Baltic and adults are generally free from the concerns of [[predation]].<ref name=":0">{{cite journal|last=Steneck|first=R. S.|title=Apex predators and trophic cascades in large marine ecosystems: Learning from serendipity|journal=Proceedings of the National Academy of Sciences|date=14 May 2012|volume=109|issue=21|pages=7953–7954|doi=10.1073/pnas.1205591109|pmid=22586126|pmc=3361373|bibcode=2012PNAS..109.7953S|doi-access=free}}</ref> Juvenile cod, however, may serve as prey for adult cod, which sometimes practice [[Cannibalism (zoology)|cannibalism]]. Juvenile cod make substrate decisions based on risk of predation. Substrates refer to different feeding and swimming environments. Without apparent risk of predation, juvenile cod demonstrated a preference for finer-grained substrates such as sand and gravel-pebble. However, in the presence of a predator, they preferred to seek safety in the space available between stones of a cobble substrate. Selection of cobble significantly reduces the risk of predation. Without access to cobble, the juvenile cod simply tries to escape a predator by fleeing.

Additionally, juvenile Atlantic cod vary their behaviour according to the foraging behaviour of predators. In the vicinity of a passive predator, cod behaviour changes very little. The juveniles prefer finer-grained substrates and otherwise avoid the safer kelp, steering clear of the predator. In contrast, in the presence of an actively foraging predator, juveniles are highly avoidant and hide in cobble or in kelp if cobble is unavailable.<ref name=Gotceitas>{{cite journal|last=Gotceitas|first=V|author2=S. Fraser |author3=J.A. Brown|title=Habitat use by juvenile Atlantic cod (Gadus morhua) in the presence of an actively foraging and non-foraging predator|journal=Marine Biology|year=1995|volume=123 | issue = 3|pages=421–430|doi=10.1007/bf00349220|bibcode=1995MarBi.123..421G|s2cid=84470358}}</ref>

Heavy fishing of cod in the 1990s and the collapse of American and [[Canadian cod]] stocks resulted in [[trophic cascade]]s. As cod are [[apex predator]]s, [[overfishing]] them removed a significant predatory pressure on other Atlantic fish and crustacean species. Population-limiting effects on several species including [[American lobster]]s, [[crab]]s, and [[shrimp]] from cod predation have decreased significantly, and the abundance of these species and their increasing range serve as evidence of the Atlantic cod's role as a major predator rather than prey.<ref name=":0" />

===Swimming===
Atlantic cod have been recorded to swim at speeds of a minimum of {{cvt|2|-|5|cm/s|kn}} and a maximum of {{cvt|21|-|54|cm/s|kn}} with a mean swimming speed of {{cvt|9|-|17|cm/s|kn}}. In one hour, cod have been recorded to cover a mean range of {{cvt|99|to|226|m}}. Swimming speed was higher during the day than at night. This is reflected in the fact that cod more actively search for food during the day. Cod likely modify their activity pattern according to the length of daylight, thus activity varies with time of year.<ref name=Lokkeborg>{{cite journal|last=LØKKEBORG|first=SVEIN|s2cid=25607249|title=Feeding behaviour of cod, Gadus morhua: activity rhythm and chemically mediated food search|journal=Animal Behaviour|date=1 August 1998|volume=56|issue=2|pages=371–378|doi=10.1006/anbe.1998.0772|pmid=9787028}}</ref>

===Response to changing temperatures===
Swimming and physiological behaviours change in response to fluctuations in water temperature. [[Respirometry]] experiments show that heart rates of Atlantic cod change drastically with changes in temperature of only a few degrees. A rise in water temperature causes marked increases in cod swimming activity. Cod typically avoid new temperature conditions, and the temperatures can dictate where they are distributed in water. They prefer to be deeper, in colder water layers during the day, and in shallower, warmer water layers at night. These fine-tuned behavioural changes to water temperature are driven by an effort to maintain homeostasis to preserve energy. This is demonstrated by the fact that a decrease of only {{convert|2.5|C-change|0}} caused a highly costly increase in metabolic rate of 15–30%.<ref name=Claireaux>{{cite journal|last=Claireaux|first=G|title=Physiology and behaviour of free-swimming Atlantic cod (Gadus morhua) facing fluctuating temperature conditions|journal=Journal of Experimental Biology|year=1995|volume=198 | issue = 1|pages=49–60|doi=10.1242/jeb.198.1.49|pmid=9317317|url=http://jeb.biologists.org/content/198/1/49.short|access-date=27 October 2013|display-authors=etal|doi-access=free|bibcode=1995JExpB.198...49C}}</ref>