Discrete spatial dynamics in a marine broadcast spawner: Re-evaluating scales of connectivity and habitat associations in Atlantic cod (Gadus morhua L.) in coastal Newfoundland

We examined the spatial dynamics of Atlantic cod, Gadus morhua, through its life history on the northeast and southeast coasts of Newfoundland in order to delineate dispersal and critical habitat requirements as potential determinants of appropriate spatial scales for management. Hydroacoustic surveys across the northeast Newfoundland shelf in 1990, 1995, and 2000 showed the distribution of the remaining large aggregations of cod to be concentrated in inshore embayments, which is contrary to historic patterns. Inshore surveys on the south coast identified local spawning aggregations at specific geographic locations, to which adults homed in subsequent years. Dispersal of eggs and larvae to the offshore occurred during spring, but was localized (within bay) in the warm summer months. Abundance of age-0 cod increased with proximity to spawning location inshore, suggesting self-recruitment within local embayments. Age-2–3 juveniles actively selected structurally complex habitat at scales of less than tens of kilometers, with home ranges typically less than 10 ha. Juveniles were associated with specific habitat types, especially at age 0, in both lab and field experiments. Our results suggest that self-recruitment and critical life-history events (e.g., spawning, settlement, and maturity) may occur within local embayments, which currently assume much greater importance to overall cod stock dynamics than in the past. Given evidence for limited dispersal, and the inshore presence of critical habitat for early life-history stages, spatial management tools that consider local dynamics may provide a greater degree of protection for coastal Newfoundland cod than previously thought.     

Growth variation and fin damage in Atlantic cod (Gadus morhua L.) fed at graded levels of feed restriction

Atlantic cod (initial weight 55, 250 and 450 g) were deprived of feed for 1 month or fed one of two diets differing in crude protein and lipid levels at rations corresponding to about 25, 50, 75 or 130% of group satiation for two months. The fish were individually weighed at the start, mid and end of the experiment and growth variation and fin damage were registered. The fin damage patterns differed between size groups; 55 g cod had most wounds on the dorsal fins whereas the pectoral fins were the most damaged in the 250 g fish. The incidence of damage in these groups was high and increased significantly when feeding was restricted. In the 450 g cod there was little fin damage and the incidence did not seem to depend on diet treatment.In the 55 and 250 g cod groups, fast growing fish had lower incidence of fin damage than fish that grew slowly, suggesting that the fish that received most aggression were prevented from feeding. However, a similar trend was registered in non-fed fish, showing that the recipients of aggression also suffered other disadvantages.The variation in individual growth rates increased when feeding was restricted and the distribution was differently skewed depending on feeding level. The data provide evidence that competition is an important factor limiting growth of individual cod held in groups in culture.     

Growth, feed efficiency, digestibility and nutrient distribution in Atlantic cod (Gadus morhua) fed two different fish meal qualities at three dietary levels of vegetable protein sources

Atlantic cod were fed six diets where the ratio of protein from fish meal to vegetable protein were varied from 91%, 67% to 46%, respectively. The experimental groups were performed in triplicate in a 20 week growth trial, increasing fish weight from about 167 g to 690 g. The vegetable protein sources constituted full-fat soybean meal and corn gluten meal at a fixed ratio of 1:2, and replaced either a high quality fish meal (DC_Mink 92.3%) or a lower quality fish meal (DC_Mink 85.6%) in the diet. All diets were extruded and balanced to be equal in gross energy, crude protein, lipid, carbohydrate, lysine and phosphorus. No difference in growth (SGR 1%) was observed comparing the two fish meal qualities. However, feed intake was significantly higher (9%) and feed efficiency lower (10%) for lower quality fish meal compared to high quality fish meal. Protein and amino acid digestibility was significantly reduced in the lower quality fish meal, while unaffected by vegetable protein inclusion for both fish meal qualities. Protein utilization as measured by protein efficiency ratio and net protein value was not affected by fish meal inclusion, except in the diet using lower quality fish meal and high vegetable protein inclusion. Lipid and energy digestibility was significantly reduced by vegetable protein inclusion for both fish meals. The lower fish meal quality increased lipid deposition in the liver and affected slaughter quality of cod by increasing HSI and dressing out percentage at low and intermediate substitution levels. Muscle composition showed small dietary changes except for arginine, while liver fatty acid composition clearly reflected fatty acid profile of full-fat soya in the diets. Corn gluten meal and full-fat soybean meal (2:1) can replace approximately 50% of dietary protein without affecting feed intake, growth, protein digestibility or slaughter quality of cod when exchanging high quality fish meal. For the lower quality fish meal diets some lower inclusion of vegetable protein sources seem to be acceptable.