Robert J. Fryer

Fish bycatch and discarding in Nephrops trawlers in the Firth of Clyde (west of Scotland)

Nephrops norvegicus(Norway lobster or prawn) is an important target species in demersal fisheries of the north-east Atlantic. Trawling for Nephrops is wasteful when many small fish are caught and discarded in the process. Here, data from 106 commercial fishing trips, sampled between 1982 and 1998 as part of the Scottish discard sampling programme, are used to investigate the fish bycatch and discards of Nephrops trawlers in the Firth of Clyde (west of Scotland). A large proportion of the fish caught in the Clyde is discarded, the discards mainly consisting of small demersal fish (mean length about 19 cm), particularly young whiting (Merlangius merlangus). Within the study period, annual estimates of total fish discards ranged from 318 to 3 027 tonnes, with a mean of 1 761 tonnes. Fish landings and discards biomass per unit effort both decreased over the study period. However, the decline in landings per unit effort was greater than that in discards per unit effort, corresponding to an increase in the discard rate over time. In recent years, discards have comprised about 70 % of the fish bycatch. The mean length of discarded fish was positively related to mesh size.

Does diurnal temperature variability affect growth in juvenile Atlantic salmon Salmo salar

This study investigated the effects of diurnal temperature variability (>7° C) on the growth of 1+ year Atlantic salmon Salmo salar. Experimental manipulation of water temperature was used to simulate: (1) constant and (2) naturally varying thermal regimes with similar daily mean values. Data from two replicates of four treatments (two thermal and two feeding regimes) were collected over 6 months corresponding to the main spring to summer growth period. Fish growth was assessed at fortnightly intervals. Small but significant differences in mean fork length (L(F) ) and mass were observed between temperature treatments, with smaller, lighter fish under the variable temperature regime. The effects of temperature regime on growth were independent of food ration. At termination of the experiment, the median L(F) and mass of fish exposed to the variable temperature regime were estimated, respectively, to be 2· 6 and 8· 0% less than those under the constant regime. Given the relatively small differences in growth attributable to variable temperature regime in these experiments, it is suggested that mean daily temperatures are adequate to inform juvenile growth models for field-based studies.

Grey seal predation impairs recovery of an over‐exploited fish stock

Grey seal predation has been blamed by fishers for the decline of Atlantic cod stocks and has led to calls for seal culls. In the West of Scotland, estimates of cod consumption by seals have exceeded reported catches and spawning biomass, focussing attention on the interaction between fishers and seals. Bayesian models making different assumptions about seal predation were used to estimate the size of the West of Scotland cod stock between 1985 and 2005 and the mortalities due to fishing and seal foraging. A simple population model was used to identify the likely direction of cod population change at recent mortality rates. All model configurations suggest that the total mortality of cod has remained fairly stable and high for many years regardless of the assumptions on seal predation. The high mortality explains the long-term decline of the stock. The best-fitting model suggests that mortality due to fishing reduced substantially in the decade up to 2005, but has been replaced by increased seal predation mortality on a smaller cod stock. Given total mortality estimates, the stock is unlikely to recover even at present reduced levels of fishing. Synthesis and applications. Our model offers a method of estimating seal predation mortality as part of routine stock assessments that inform fishery management. The analysis shows that predation by seals can be an important component of the total stock mortality. It also shows that assuming invariant natural mortality, as adopted in many standard fish stock assessments, may lead to incorrect perceptions of fishing mortality, over-estimating the benefits of reducing fishing mortality when there is density-dependent predation. It is essential to consider predation by top predators when formulating appropriate advice for managing the fishery.

Development of spatial regression models for predicting summer river temperatures from landscape characteristics: implications for land and fisheries management

There is increasing demand for models that can accurately predict river temperature at the large spatial scales appropriate to river management. This paper combined summer water temperature data from a strategically designed, quality controlled network of 25 sites, with recently developed flexible spatial regression models, to understand and predict river temperature across a 3000 km2 river catchment. Minimum, mean and maximum temperatures were modelled as a function of nine potential landscape covariates that represented proxies for heat and water exchange processes. Generalised Additive Models were used to allow for flexible responses. Spatial structure in the river network data (local spatial variation) was accounted for by including river network smoothers. Minimum and mean temperatures decreased with increasing elevation, riparian woodland and channel gradient. Maximum temperatures increased with channel width. There was greater between-river and between-reach variability in all temperature metrics in lower order rivers indicating that increased monitoring effort should be focussed at these smaller scales. The combination of strategic network design and recently developed spatial statistical approaches employed in this study have not been used in previous studies of river temperature. The resulting catchment scale temperature models provide a valuable quantitative tool for understanding and predicting river temperature variability at the catchment scales relevant to land use planning and fisheries management and provide a template for future studies.

Can Conservation Stocking Enhance Juvenile Emigrant Production in Wild Atlantic Salmon

AbstractConservation stocking is frequently used by fishery managers to stabilize or increase production of depleted fish stocks. However, in the case of salmonids the benefits are increasingly questioned and generally poorly quantified. We investigated the effects of ova stocking on freshwater emigrant production in a declining Scottish population of “spring” Atlantic Salmon Salmo salar. The stocking program was designed to minimize risks, maximize expected benefits, and bypass population bottlenecks between the ova and fry stages. Long-term data (33 cohorts over 42 years, including 8 years of stocking) on numbers of ova and juvenile emigrant production were used to investigate whether the ova–emigrant stock–recruitment relationship differed between conditions of natural spawning and stocking. We considered Ricker and Beverton–Holt models that included terms for the effects of stocking, intercohort competition, and changes in trap efficiency on emigrant production. The “best model” was considered to be t...

Is Interstitial Velocity a Good Predictor of Salmonid Embryo Survival

Abstract Interstitial velocity within spawning bed substrates is frequently used to predict salmonid embryo survival and often forms the basis of habitat assessment or sediment-based predictive models. In this study, the relationships between interstitial velocity, hyporheic dissolved oxygen (DO), and survival of Atlantic salmon Salmo salar embryos were examined in an experimental field study that incorporated natural complexity in the hyporheic environment. Hyporheic DO and interstitial velocity were not clearly related over the observed range of velocities. There was strong evidence of a relationship between survival and DO but only weak evidence of a relationship with interstitial velocity. Conceptual models of hyporheic environmental variation and salmonid embryo survival should include the appropriate range of environmental processes. Given the findings of this study, we suggest that interstitial velocity should not be used to assess or predict spawning habitat quality; direct measurements of DO offe...

Population variation in thermal growth responses of juvenile Atlantic cod (Gadus morhua L.)

Controlled environment experiments were carried out to investigate thermal influences and population differences on growth of wild-caught juvenile Atlantic cod Gadus morhua L. from two regions of differing thermal regime off Scotland; the Clyde Sea on the west coast and St Andrews Bay on the east coast. Cod from the Clyde demonstrated significantly higher growth rates than cod from St Andrews. In both populations the growth rate was greater at 12°C than at 8°C. These population and temperature effects act to reinforce one another and it could therefore be predicted that the growth differences between the two areas in the wild should be even more pronounced. The results are consistent with the suggestion that cod may be locally adapted to their thermal environment.

Effects of individual variation in length, condition and run-time on return rates of wild-reared Atlantic salmon Salmo salar smolts: survival of salmo salar

Groups of wild-reared Atlantic salmon Salmo salar smolts were captured during their seaward migration on a tributary of the River Conon, Scotland, U.K., from 1999 to 2014 and tagged with passive integrated transponders (PIT). Fish that subsequently returned to the river after growing at sea were recorded automatically by a PIT-detector in a fish pass. Return rate was related directly to length and condition and inversely to day of the year that the smolt was tagged. Over years, as the study progressed, there was a significant increase in the proportion of smolts returning after two or more years at sea and no trend in returns of salmon having spent one winter at sea. There was no trend in the date of return of salmon across the study period. Fish that had spent more winters at sea returned earlier in the year.