Justin J. Meager

Effects of turbidity on the spontaneous and prey-searching activity of juvenile Atlantic cod (Gadus morhua)

Increasing turbidity in coastal waters in the North Atlantic and adjacent seas has raised concerns about impacts on Atlantic cod (Gadus morhua) using these areas as nurseries. A previous experiment (Meager et al. 2005 Can. J. Fish. Aquat. Sci. 62, 1978–1984) has shown that turbidity (up to 28 beam attenuation m−1) had little effect on the foraging rate of juvenile cod. Although this was attributed to cod using chemoreception in conjunction with vision to locate prey, foraging rates may also be maintained by increased activity. Higher activity, however, is energetically costly and may offset benefits from increased foraging return. We examined the effects of turbidity on prey searching and spontaneous activity of juvenile cod in the laboratory, by measuring activity with and without prey cues. Activity of juvenile cod was nonlinearly affected by turbidity and was lower at intermediate turbidity, regardless of the presence of prey odour. Activity increased over time when prey odour was present and decreased when absent, but the effects of prey odour were similar across all turbidity levels. Position in the tank was unaffected by turbidity or prey odour. Reduced activity at intermediate turbidities is likely to offset longer prey-search times. At high turbidity (greater than 17 m−1), both longer prey-search times and higher activity indicate that increased energetic costs are probable.

Domestication causes rapid changes in heart and brain morphology in Atlantic cod (Gadus morhua)

Brain and heart development is very plastic in teleost fishes, and receptive to changes in social and environmental conditions. Domestication in salmonids has been reported to result in pronounced changes in both heart and brain morphology. In particular, a high prevalence of heart deformities has been reported in farmed salmonids, which has been linked to increased stress responsiveness that can impair survival of both farmed and escaped fish. Here we report for the first time that significant changes in heart and brain morphology occur following domestication of Atlantic cod (Gadus morhua), an emerging aquaculture species. Juvenile farmed cod developed significantly larger hearts and smaller brains, by weight, compared to their wild conspecifics. These differences occurred within the first captive generation, suggesting that they were driven largely by the strong contrast in environmental and social conditions experienced within their respective rearing environments. Changes in brain and heart morphology, as a consequence of domestication could affect the well-being and survival of Atlantic cod raised under intensive aquaculture conditions.

Reproductive interactions between fugitive farmed and wild Atlantic cod (Gadus morhua) in the field

Atlantic cod (Gadus morhua) are being increasingly farmed in net pens adjacent to coastal populations that are currently at historic lows. One concern is that farmed escapees enter local spawning shoals and mate with wild cod. We tested for the potential of escaped farmed cod to interact and hybridize with wild fish by examining the spatial dynamics of, and associations between, fish tagged with ultrasonic transmitters. Based on these data, we also investigated the basic mating system of cod in the field. The spawning ground was best described as a lekking arena. Wild males aggregated near the seafloor and associations between individuals were frequent. Wild females had a pelagic and dispersed distribution and rarely associated with each other. Associations between individual wild males and females were also infrequent. Farmed males rarely associated with wild fish and had core usage areas above the wild males, suggesting that they were not admitted into the spawning arena. Farmed females were over the sp...

Effect of turbidity on habitat preference of juvenile Atlantic cod, Gadus morhua

We examined the effects of turbidity on habitat preference of juvenile Atlantic cod in the laboratory, using a shuttle box where fish could select between two different habitats. In the first experiment, we compared three turbidity levels of kaolin (3, 8 and 21 beam attenuation m−1). In the second experiment, we looked at the effect of turbidity media (kaolin versus algae), after controlling for spectral differences between turbidity media. Although cod preferred an intermediate turbidity of kaolin over low turbidity water, comparisons between low and high turbidity, and intermediate and high turbidity did not significantly influence habitat preference. Algae did not influence habitat preference by cod. Although other studies have found that turbidity affects both foraging and antipredator behaviour of juvenile cod, this study has shown that gradients in turbidity per se do not have a strong effect on their habitat preference.

Multidimensionality of behavioural phenotypes in Atlantic cod, Gadus morhua

Much of the inter-individual variation observed in animal behaviour is now attributed to the existence of behavioural phenotypes or animal personalities. Such phenotypes may be fundamental to fisheries and aquaculture, yet there have been few detailed studies of this phenomenon in exploited marine animals. We investigated the behavioural and neuroendocrine responses of Atlantic cod (Gadus morhua L.), to situations reflecting critical ecological challenges: predator attacks and territorial challenges. Both hatchery-reared and wild fish were tested and behavioural profiles were compared with baseline conditions. We then used an objective, multivariate approach, rather than assigning individuals along one-dimensional behavioural axes, to examine whether distinct behavioural phenotypes were present. Our results indicate that two distinct behavioural phenotypes were evident in fish from each background. In hatchery-reared fish, phenotypes displayed divergent locomotor activity, sheltering, brain monoamine concentrations and responses to competitive challenges. In wild fish, phenotypes were distinguished primarily by locomotor activity, sheltering and responsiveness to predator stimuli. Hatcheries presumably represent a more stressful social environment, and social behaviour and neuroendocrine responses were important in discerning behavioural phenotypes in hatchery fish, whereas antipredator responses were important in discerning phenotypes in wild fish that have previously encountered predators. In both fish types, behavioural and physiological traits that classified individuals into phenotypes were not the same as those that were correlated across situations. These results highlight the multidimensionality of animal personalities, and that the processes that regulate one suite of behavioural traits may be very different to the processes that regulate other behaviours.

Green turtle (Chelonia mydas) population demographics at three chemically distinct foraging areas in the northern Great Barrier Reef

The catchments of the Great Barrier Reef (GBR) have experienced significant modifications in recent decades, leading to increases in sources of pollutants and declines in coastal water quality. As coastal waters of the GBR support some of the highest density green turtle (Chelonia mydas) foraging populations in the western Pacific Ocean, understanding the effects of contaminants on GBR green turtle populations is a priority. In 2012, elevated strandings of green turtles in the Upstart Bay region instigated the WWFs collaborative Rivers to Reef to Turtles (RRT) project to investigate if coastal pollutants are compromising green turtle health. Important to interpreting these investigations into toxicology and health is understanding the demographics of the green turtle populations being investigated. In three green turtle foraging grounds, Cleveland Bay (CLV), Upstart Bay (UPB) and the Howick Group of Reefs (HWK), this study explored population size, age class structure, sex ratio, growth rates, body condition and diet, as well as indices of turtle health, such as plastron barnacle loads and eye lesions. The three foraging populations had similar age class structure and adult sex ratios to other green turtle foraging populations in the GBR. Somatic growth rate was nonlinear, peaking in immature turtles, and was much slower in turtles foraging at HWK compared to the other two sites. This may have been due to differences in food source, which was supported by the observed dietary shifts between seagrass and algae in HWK turtles, compared to a consistently seagrass diet in CLV and UPB turtles. There were also small differences in body condition between sites, as well as differences in barnacle loads, eye lesions and occurrence of fibropapilloma tumors. This study provides important information on green turtle foraging ground population dynamics in the northern GBR, and context for the other papers in this special issue.

The behavioural diversity of Atlantic cod: insights into variability within and between individuals

Cod (Gadus morhua) are an iconic fish species of cultural, historical and economical significance across the Atlantic and adjacent seas. Among many scholarly investigations, this interest has prompted behavioural research, rendering cod one of the few commercially harvested marine fishes for which behaviour has been studied in a comprehensive manner. In our review of this behavioural work, we examine the variability in cod behaviour across five functional domains: foraging, predation, social interactions, migration and reproduction. Research to date suggests a high level of behavioural sophistication in cod that is underpinned by complex learning strategies and long-term memory. Cod also demonstrate substantial variability in how they respond to different ecological circumstances. Considerable variation is evident both within and between individuals, and in some instances, between populations. There are a number of pathways from which this variation appears to arise, such as asocial and social learning, environmental control of phenotypic plasticity and genetic control, but there are no known examples of behaviours that are purely the result of one of these mechanisms. Behavioural variation is therefore likely to result from a combination of these factors, underscoring the need for a quantitative, multivariate approach to understand behavioural variation in cod.