Behaviour, resource use, and condition of demersal age 0 Atlantic cod (Gadus morhua) in coastal Newfoundland : components of survival risk in the nearshore environment

Grant, Scott Michael (2000) Behaviour, resource use, and condition of demersal age 0 Atlantic cod (Gadus morhua) in coastal Newfoundland : components of survival risk in the nearshore environment. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

It is well established, that following a pelagic phase age 0 Atlantic cod (Gadus morhua) settle to the bottom to assume a demersal habit. However, risks to juvenile cod survival in the coastal zone of the northwest Atlantic are not well understood. A better knowledge of factors affecting survival of age 0 cod during the transition to a demersal habit in shallow (< 10 m deep) nearshore regions of Newfoundland has broad ecological applications and will improve our understanding of the role of coastal habitats to the rebuilding of north Atlantic cod stocks. In this thesis, I investigated the importance of nearshore regions in coastal Newfoundland as rearing grounds for age 0 cod by examining their behaviour, resource use, and condition in relation to diet for the first several months post-settlement. -- Day-night differences in catch and diet of age 0 and age 1 cod were examined to delineate the diel peak in foraging activity and resource use in eelgrass (Zostera marina) habitat. Cod exhibited a size-related shift (age 0 to 1) from feeding predominantly on zooplankton by day to benthos at night. Intercohort cannibalism occurred when age 1 cod were approximately three times larger than their prey. Diet composition and catch data revealed that concentrations of age 0 cod foraged in the water column during the day, ceased feeding at night, and appeared to move to the bottom. Seasonally decreasing day length and prey size contributed to a decline in daily ration, yet age 0 cod maintained a strictly diurnal foraging cycle. The nocturnal decrease in feeding coincided with an increase in the catch of older conspecifics (age 2 and 3) and increased foraging activity of age I cod suggesting that avoidance of older conspecifics is an important factor influencing the survival of age 0 cod in the nearshore. -- Capture-mark-recapture experiments and seasonal sampling were conducted to examine the movement levels, spatial distribution, behaviour, and abundance of age 0 cod in eelgrass and no-eelgrass habitat over two years. Cod remained localized for several weeks (September-December) after settling from a pelagic habit and may remain localized during their first winter. Marked cod did not appear to move further than a few hundred metres within eelgrass habitat, but moved up to 1.2 km in no-eelgrass habitat. Slope values for variance-mean density relationships indicate that age 0 cod aggregated during the day, while high within and between site variation in catch at the scale of hours and weeks provide evidence of localized lateral movements. These patterns in the catch rates are interpreted to reflect social behaviour in the form of mobile shoaling aggregations during the day. Day- night comparisons of ratios of variance to mean density were also indicative of aggregative behaviour during the day. Day-night patterns in the catch rates are interpreted to be due to a decrease in activity and local dispersal of shoaling aggregations at night. A significant increase in abundance of age 0 cod in less suitable habitat (no-eelgrass) when settlement strength was high is consistent with the hypothesis of density dependent habitat selection. -- Diet composition and simple indices of condition were compared during the autumnal settlement of age 0 cod. Laboratory experiments were also conducted to determine whether simple indices of condition reflect differences in energy intake (ration level) and to determine minimum index of condition values for age 0 cod that died due to exhaustion of their energy reserves. Results agreed with published literature on the value of simple indices to assess condition of cod. Liver and contractile muscle energy reserves of wild age 0 cod varied seasonally, however, their condition was well above that of cod that died in the laboratory. Laboratory studies revealed that condition of cod differed significantly over time and between daily ration levels. Indices of condition representing variation in muscle energy reserves exhibited a steady decline once the hepto-somatic index fell below a value of about 2.0, which I interpret as evidence of a pattern of depletion of lipids and then proteins, similar to that observed for larger juveniles and adults. I conclude that as long as age 0 cod are above a safe level of condition their body length may still increase despite a decline in condition. -- Condition of a laboratory cultured and wild juvenile cod population from coastal Newfoundland were compared to determine whether small juveniles develop enlarged fatty livers in captivity. Cultured cod developed enlarged fatty livers, exhibiting significantly higher liver and contractile muscle energy reserves than wild cod. Cultured cod of similar age exhibited a high divergence in body size over time. Indices of liver energy reserves were positively correlated with body size suggesting that a size-related social hierarchy had developed in the laboratory. In the wild, indices of liver energy reserves were negatively correlated with body size, suggesting high utilization of dietary lipids by larger juveniles in association with increased metabolic expenditure when feeding on small prey items. -- I conclude, from studies of condition in relation to diet, that shallow water coastal environments may serve as trophically adequate rearing grounds for age 0 cod for the first several months post-settlement. Behaviour patterns exhibited by age 0 cod, i.e., restricted movements in shallow nearshore waters, diurnal shoaling, and preference for structurally complex habitat, are mechanisms for predator avoidance, suggesting that predation risk is high in coastal Newfoundland. Predation risk, particularly from older conspecifics, in the nearshore environment and its potential to influence cod recruitment warrants further investigation.

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