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Sea cage fish aquaculture attracts large aggregations of wild fish that feed on farm waste. Fish that associate closely with farms undergo physiological changes, and captive feeding trials indicate possible negative effects on reproductive fitness. However, little is known about the significance of this phenomenon for reproduction in wild fish over larger spatial scales. To assess whether coastal areas with intensive aquaculture impact the fitness of wild fish, we collected Atlantic cod Gadus morhua from 2 areas, 1 with high and 1 with low Atlantic salmon Salmo salar farming density (HFD and LFD, respectively) in southwestern Norway, a region that hosts the world’s largest coastal fish aquaculture industry. We conducted a captive spawning trial and compared a range of reproductive fitness metrics. Two fatty acids that occur at high levels in commercial feed, oleic and lineoleic acid, were strongly correlated in the ovaries of HFD fish, but a comparison of lipid profiles between HFD and LFD fish showed no differences in total lipids or essential fatty acids. Although HFD fish were slightly larger than LFD fish and had similar body condition, the volume of eggs produced did not differ, indicating relatively lower reproductive investment by HFD fish. HFD eggs were 5% smaller than LFD eggs, which did not lead to differential hatching success but may have contributed to HFD larvae being 8% smaller than their LFD counterparts at 40 d post spawning. The potential for cumulative effects of smaller eggs and larvae on fitness at later life stages warrants further investigation.

Genetic interactions between farmed escapees and wild fish represent a challenge to environmentally sustainable aquaculture. Breeding programs for Atlantic cod Gadus morhua have been initiated; however, the genetic response to selection, and therefore the degree of domestication, has not been evaluated. We compared growth of 2 wild and 2 partly domesticated strains that had been under selection for 2 generations. Offspring of 54 synchronously produced families were reared in 2 common-garden experiments, each consisting of Phase I: parallel rearing in mesocosms and tanks 0-8 mo post-hatch, and Phase II: rearing in tanks or sea-cages 8-18 and 8-34 mo post-hatch, respectively. One of the domesticated strains displayed significantly higher growth compared to the wild Northeast Arctic cod population (48-67% higher weight), while the other domesticated strain had a similar growth rate to the Northeast Arctic cod population. The wild population from southern Norway displayed a significantly higher growth rate compared to the wild Northeast Arctic cod population. These results represent the first experimental estimation of domestication-driven changes in farmed cod, and demonstrate that the first breeding programs for this species have been partially successful, resulting in improved growth rates of cod in 2 generations.

Sea trout face growth-mortality trade-offs when entering the sea to feed. Salmon lice epizootics resulting from aquaculture have shifted these trade-offs, as salmon lice might both increase mortality and reduce growth of sea trout. We studied mortality and behavioural adaptations of wild sea trout in a large-scale experiment with acoustic telemetry in an aquaculture intensive area that was fallowed (emptied of fish) synchronically biannually, creating large variations in salmon lice concentrations. We tagged 310 wild sea trout during 3 years, and gave half of the individuals a prophylaxis against further salmon lice infestation. There was no difference in survival among years or between treatments. In years of high infestation pressure, however, sea trout remained closer to the river outlet, used freshwater (FW) habitats for longer periods and returned earlier to the river than in the low infestation year. This indicates that sea trout adapt their migratory behaviour by actively choosing FW refuges from salmon lice to escape from immediate mortality risk. Nevertheless, simulations show that these adaptations can lead to lost growth opportunities. Reduced growth can increase long-term mortality of sea trout due to prolonged exposure to size-dependent predation risk, lead to lower fecundity and, ultimately, reduce the likelihood of sea migration. [ABSTRACT FROM AUTHOR]

Wild caught North Sea saithe Pollachius virens were monitored for growth, sex steroid profiles and oocyte development pre-spawning and measured for egg size and group fecundity during the spawning season in the laboratory. Vitellogenesis commenced in late October-early November, at a leading cohort size ( CL) of c. 250 µm, after which oocytes grew rapidly in size until spawning started in February. Notably, a distinct cortical alveoli stage was virtually absent with yolk granules observed in developing oocytes at the very beginning of vitellogenesis. Little atresia was observed pre-spawning, but atretic re-absorption of remnant oocytes containing yolk granules was found in all females immediately post-spawning. As expected, concentrations of sex steroids, oestradiol-17β (females), testosterone (both sexes) and 11-ketotestosterone (both sexes), increased pre-spawning before dropping post-spawning. The present experiment provides the first validation of sex steroid levels in P. virens. Post-ovulatory follicles were visible in histological sections from female gonads 9-11 months post-spawning, but then disappeared. Spawning commenced around a CL of c. 750 µm (700-800 µm). Hydrated oocytes (eggs) measured between 1·04 and 1·31 mm (mean = 1·18 mm) with decreasing sizes towards the end of spawning. The average estimated realized fecundity was c. 0·84 million eggs (median female total length, LT = 60 cm). Spawning lasted from 13 February to 29 March. [ABSTRACT FROM AUTHOR]