Spatial risk: how predator distributions shape habitat use by age-0 cod

MacRobert, Evelyn (2020) Spatial risk: how predator distributions shape habitat use by age-0 cod. Masters thesis, Memorial University of Newfoundland.

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Abstract

The early life history stages of fish experience greater mortality rates than older conspecifics, primarily as a result of predation. Determining which habitats pose the highest predation “risk” can help in understanding habitat use by juvenile cod. In two field experiments, I examined spatial and temporal segregation of age-0 Atlantic (Gadus morhua) and Greenland (Gadus macrocephalus ogac) cod from piscivorous fishes. In Chapter 2, I used underwater video footage to construct a habitat map and locate age-0 Atlantic and Greenland cod in Newman Sound, Bonavista Bay, Newfoundland. Age-0 cod significantly associated with uniform eelgrass habitat, as opposed to less use of kelp or bare substrates, with all individuals observed within 1 m of eelgrass patches. In Chapter 3, I tested seasonal and diel habitat associations of 37 individuals of four known piscivorous fish species – age-1+ Atlantic and Greenland cod, cunner (Tautogolabrus adspersus), and shorthorn sculpin (Myoxocephalus scorpius) – using passive acoustic telemetry. Habitat use differed significantly among species. Season significantly affected habitat use for Greenland cod, cunner, and sculpin, whereas diel period significantly influenced habitat use for cunner. Greenland cod and cunner preferred eelgrass in the spring, then transitioned to deeper water habitats (kelp, sparse vegetation, and bare substrates) during summer and fall. Overall, piscivores primarily occupied shallow habitats in spring, and transitioned to deeper water habitats during summer and fall, outside the eelgrass beds frequented by age-0 cod. My thesis demonstrates spatial and temporal segregation of age-0 cod into nursery eelgrass habitat, apart from their predators.

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