Narrowing the Range of Environmental Salinities Where Juvenile Meagre (Argyrosomus regius) Can Be Cultured Based on an Osmoregulatory Pilot Study

Survival rates and physiological recovery responses in the lesser-spotted catshark (Scyliorhinus canicula) after bottom-trawling

In 2019, Europe will adopt a no-discards policy in fisheries. This entails the landing of captured species unless strong evidence is provided supporting their survival and recovery after fishing. Thus, research on this topic is gaining momentum. Bottom-trawling, as a non-selective fishing method, is characterized by a high proportion of discards including vulnerable key species, such as demersal sharks. Their survival may also depend on capture depth. By paralleling onboard and laboratory experiments with the small-spotted catshark, Scyliorhinus canicula, we offer a robust experimental design to assess the survival of discarded sharks. Catsharks were captured by bottom-trawling at two depths (shallow ~89 m and deep ~479 m). Blood samples were collected following trawl capture and analyzed for stress biomarkers (lactate, osmolality, phosphate, urea). During recovery in onboard tanks, behavior was video-recorded and fish were re-sampled after 24 h. A second experiment was conducted in laboratory facilities to simulate air-exposure after trawling and to analyze the physiological recovery. Our results showed that 95.7% of the animals survived 24 h after trawling. We confirmed that trawling elicited acute stress responses in catshark but that they managed to recover. This was demonstrated by lactate concentrations that were 2.6 mM upon capture, but recovered to assumed baselines after 24 h (0.2 mM). Non-invasive video monitoring revealed behavioral differences with depth, whereby those captured at 89 m depth required longer to recover than those captured at 479 m depth. Implementation of standardized survival studies by fishery managers can benefit from holistic physiological approaches, such as the one proposed here.

Environmental Salinity Affects Growth and Metabolism in Fingerling Meagre (Argyrosomus Regius)

The meagre (Argyrosomus regius), a farmed fish in Mediterranean countries, seasonally migrates from offshore areas to estuaries for reproduction. During the first two years of life, the meagre evidences a certain grade of euryhalinity by staying in brackish waters close to the shore. The aim of the present study was to establish if fingerling growth in brackish water is improved compared to seawater, where current culture procedures are conducted. Three-month-old fingerlings were maintained for 45 days under two different salinity regimens (12 and 39 ppt). Several growth parameters as well as osmoregulatory and metabolic variables were assessed. Specific growth rate and hepatosomatic index values revealed that fingerlings performed better in brackish waters (12 ppt) compared to 39 ppt. This study contributes to optimizing meagre rearing conditions, thereby supporting the role of A. regius in aquaculture diversification.

Evidence of euryhalinity of the Gulf corvina (Cynoscion othonopterus)

The effects of environmental salinity on physiological responses, growth, and survival of the Gulf corvina, C. othonopterus, were evaluated in a 6-week completely randomized design experiment. Corvina (17.2+/-2.3 g mean initial body weight) were subjected to salinities of 5, 15, 25, and 35 ‰ and fed a commercial feed with protein and lipid contents of 46 and 14 %, respectively. Plasma osmolality increased significantly with salinity, ranging from 335.1+/-5.3 mOsm/kg in fish maintained at 5 ‰, to 354.8+/-6.8 mOsm/kg in fish kept in seawater, while a significant inverse relationship was observed between salinity and moisture content of whole fish, ranging from 73.8+/-0.7 (measured at 5 ‰) to 76.9+/-1.0 % (measured at 35 ‰). In spite of this, growth indices (final weight, weight gain, specific growth rate, condition factor, survival) were not altered, suggesting that, like other members of the family Sciaenidae, the Gulf corvina is a strong osmoregulator. The isosmotic point for this species was estimated to correspond to a salinity of 9.8 ‰. The present study represents the first set of experimental data on salinity tolerance of C. othonopterus and confirms the euryhalinity of this species.