Adriaan D. Rijnsdorp

Reconsidering the Consequences of Selective Fisheries

Balanced fishing across a range of species, stocks, and sizes could mitigate adverse effects and address food security better than increased selectivity. Concern about the impact of fishing on ecosystems and fisheries production is increasing (1, 2). Strategies to reduce these impacts while addressing the growing need for food security (3) include increasing selectivity (1, 2): capturing species, sexes, and sizes in proportions that differ from their occurrence in the ecosystem. Increasing evidence suggests that more selective fishing neither maximizes production nor minimizes impacts (4–7). Balanced harvesting would more effectively mitigate adverse ecological effects of fishing while supporting sustainable fisheries. This strategy, which challenges present management paradigms, distributes a moderate mortality from fishing across the widest possible range of species, stocks, and sizes in an ecosystem, in proportion to their natural productivity (8), so that the relative size and species composition is maintained.

Recruitment of sole stocks, Solea solea (L.), in the Northeast Atlantic

Abstract The level of recruitment and its variability were analysed in sole stocks using results of virtual population analysis (VPA) of commercial landings and pre-recruit surveys. Stocks were studied in the North Sea, eastern and western English Channel, Bristol Channel and the Irish Sea, Variability of recruitment increased from 34–55% in the central areas to 97–127% in the northern ones: the Irish Sea and the North Sea. Winter temperatures explained part of the additional variability in recruitment in the North Sea but not in the Irish Sea. In the North Sea level of recruitment appeared to be determined before the end of the first year of life, although additional mortality of older fish during cold winters may further reduce recruitment. In general there was no similarity in the pattern of recruitment variability between populations, except in adjacent populations, suggesting that the factors determining recruitment differ substantially over rather small distances. The level of recruitment in the five populations was related to the surface area of the nursery grounds.

Selective Tidal Transport of North Sea Plaice Larvae Pleuronectes platessa in Coastal Nursery Areas

Abstract The transport of plaice larvae from the open sea towards shallow, mainly inshore, nursery areas along the North Sea coast was studied for 5 years in the Easter Scheldt and the Wadden Sea. Metamorphosing larvae (stages 4 and 5) entered the nursery areas in March and April. Larva density was significantly correlated with the density of 0-group plaice at the end of the following summer. Significant factors determining the larval densities were date of sampling, location, tide, depth, and did period. Larvae usually were most abundant in the bottom stratum, but during nighttime flood tides they moved into midwater and surface strata. Flood catches exceeded the ebb catches of pelagic larvae for all stations. The results suggest that plaice larvae accomplish passive but selective horizontal transport by swimming up from the seabed during flood tides and remaining on the seabed during ebb tides.

Low effective population size and evidence for inbreeding in an overexploited flatfish, plaice (Pleuronectes platessa L.)

Overexploitation and subsequent collapse of major worldwide fisheries has made it clear that marine stocks are not inexhaustible. Unfortunately, the perception remains that marine fishes are resilient to large population reductions, as even a commercially ‘collapsed’ stock will still consist of millions of individuals. Coupled with this notion is the idea that fisheries can, therefore, have little effect on the genetic diversity of stocks. We used DNA from archived otoliths collected between 1924 and 1972 together with 2002 juveniles tissue to estimate effective population size (Ne) in plaice (Pleuronectes platessa). Ne was estimated at 20 000 in the North Sea and 2000 in Iceland. These values are five orders of magnitude smaller than the estimated census size for the two locations. Populations examined between 1924 and 1960 were in Hardy–Weinberg equilibrium, whereas populations examined after approximately 1970 were not. Extensive testing was performed to rule out genotyping artefacts and Wahlund effects. The significant heterozygote deficiencies found from 1970 onward were attributed to inbreeding. The emergence of inbreeding between 1950 and 1970 coincides with the increase in fishing mortality after World War II. Although the biological mechanisms remain speculative, our demonstration of inbreeding signals the need for understanding the social and mating behaviour in commercially important fishes.

Population structure of plaice (Pleuronectes platessa L.) in northern Europe: microsatellites revealed large-scale spatial and temporal homogeneity

Philopatry to spawning grounds combined with well‐known migratory patterns in the flatfish Pleuronectes platessa (plaice) has led to the hypothesis that regional populations may reflect relatively discrete, genetic stocks. Using six microsatellite loci we genotyped 240 adult individuals collected from locations in Norway, the Faeroe plateau, the Irish Sea, the Femer Baelt, Denmark, and the southern North Sea, and 240 0‐class juveniles collected from five nursery‐ground locations in Iceland, northwest Scotland, two sites in the Wadden Sea, and the Bay of Vilaine in Southern Brittany. The mean number of alleles/locus ranged from 5.3 to 20.4, with a mean of 13.9. Expected heterozygosity was uniformly high across all locations (multilocus Hexp = 0.744 ± 0.02). Pairwise comparisons of θ among all 11 locations revealed significant differentiation between Iceland and all other locations (θ = 0.0290*** to 0.0456***), which is consistent with the deep–water barrier to dispersal in plaice. In contrast, no significant differentiation was found among any of the remaining continental‐shelf sampling locations. This suggests that regional stocks are themselves composed of several genetic stocks under a model of panmixia which persists even to the spawning grounds. The presence of significant heterozygote deficiencies at all locations (not due to null alleles) suggests a temporal Wahlund effect yet the absence of significant population differentiation among continental shelf localities makes this explanation alone, difficult to reconcile. Sampling of eggs at the spawning grounds will be required to resolve this issue. Causes of the mismatch between genetic and geographical stocks is discussed in the context of high gene flow.

Phylogeography and population structure of thornback rays (Raja clavata L., Rajidae)

The phylogeography of thornback rays (Raja clavata) was assessed from European waters, using five nuclear microsatellite loci and mitochondrial cytochome b sequences. Strong regional differentiation was found between the Mediterranean basin, the Azores and the European continental shelf. Allelic and haplotype diversities were high in Portuguese populations, consistent with the existence of a refugium along the Iberian Peninsula. Unexpectedly, high diversity was also found in the English Channel/North Sea area. The lowest genetic diversity was found in the Black Sea. Populations sampled from the Mediterranean, Adriatic and Black Seas were characterized by a single mitochondrial haplotype. This haplotype was also the most ancestral and widespread outside of the Mediterranean basin except for the Azores. Populations from the Azores were dominated by a second ancestral haplotype which was shared with British populations. Results from multidimensional scaling, amova and nested clade analysis indicate that British waters are a secondary contact zone recolonized from at least two refugia — one around the Iberian Peninsula and one possibly in the Azores. Links to a potential refugium known as the Hurd Deep, between Cornwall and Brittany, are discussed. Finally, a historical demographic analysis indicates that thornback ray populations started to expand between 580 000 and 362 000 years ago, which suggests that the Last Glacial Maximum (20 000 years ago) had mainly affected the distribution of populations rather than population size.

On the survival of plaice and sole discards in the otter-trawl and beam-trawl fisheries in the North Sea

Abstract This paper summarizes the results of survival experiments with plaice and sole discards caught by commercial beam-trawl vessels and a research vessel fishing with an otter trawl, and with soles escaped through the meshes in covered-cod-end experiments. In the commercial beam-trawl fishery the survival of both plaice and sole discards was estimated to be less than 10%. The survival of soles escaped through the meshes was estimated at 60%. The survival of discards was negatively affected by the catching process. During the catching process mortality was caused through the action of the tickler chains and the injuries imposed during the stay in the net. Under present-day commercial practice the processing of the catch on deck hardly affects the survival of discards.

Impact of juvenile growth on recruitment in flatfish

Abstract In this review, the impact of juvenile growth on subsequent recruitment in flatfish is discussed. Recruitment is defined as the number of specimens of a specific year class that survives to attain sexual maturity and joins the reproductive population. Theoretically, variability in growth rate can have an impact on recruitment either by mean of size-selective mortality during juvenile life and/or by means of size-dependent onset of maturation. In flatfish up to about 10 cm, growth depends on size in such a way that variability in size within a population increases during the first year of life, and decreases again in the subsequent part of juvenile life. Temporal variability in size within local populations appears to be lower than spatial variability. Due to the prolonged spawning period, and hence period of settlement, variability in size of juvenile flatfish increases with decreasing latitude. As a consequence of these patterns, size-selective mortality appears to be mainly restricted to the 0-group and to gain importance with decreasing latitude. A literature search for field data yielded only a few references suggesting size-selective mortality. In none of the studies was any relationship with ultimate recruitment studied or even suggested. Size-dependent onset of maturation has been found in some flatfish species, with slow-growing individuals or cohorts showing delayed maturation. Size-dependent onset of maturation has a clear effect on the level of recruitment. However, in the species studied, the main traits in year-class strength still existed at the moment of recruitment to the reproducing stock. Size-dependent onset of maturation also appeared to affect the year-to-year variability in recruitment, but different effects were observed among species..It is argued that both size-selective mortality and size-dependent onset of maturation are more likely to dampen than to generate variability in recruitment. The study of the impact of juvenile growth on recruitment in flatfish is hampered by the absence of long-term data sets on recruitment. Especially comparable series of (sub)tropical species and of populations covering the total range of distribution of a species are lacking.

Evolutionary impact assessment: accounting for evolutionary consequences of fishing in an ecosystem approach to fisheries management

Managing fisheries resources to maintain healthy ecosystems is one of the main goals of the ecosystem approach to fisheries (EAF). While a number of international treaties call for the implementation of EAF, there are still gaps in the underlying methodology. One aspect that has received substantial scientific attention recently is fisheries-induced evolution (FIE). Increasing evidence indicates that intensive fishing has the potential to exert strong directional selection on life-history traits, behaviour, physiology, and morphology of exploited fish. Of particular concern is that reversing evolutionary responses to fishing can be much more difficult than reversing demographic or phenotypically plastic responses. Furthermore, like climate change, multiple agents cause FIE, with effects accumulating over time. Consequently, FIE may alter the utility derived from fish stocks, which in turn can modify the monetary value living aquatic resources provide to society. Quantifying and predicting the evolutionary effects of fishing is therefore important for both ecological and economic reasons. An important reason this is not happening is the lack of an appropriate assessment framework. We therefore describe the evolutionary impact assessment (EvoIA) as a structured approach for assessing the evolutionary consequences of fishing and evaluating the predicted evolutionary outcomes of alternative management options. EvoIA can contribute to EAF by clarifying how evolution may alter stock properties and ecological relations, support the precautionary approach to fisheries management by addressing a previously overlooked source of uncertainty and risk, and thus contribute to sustainable fisheries.

Changes in growth of plaice Pleuronectes platessa L. and sole Solea solea (L.) in the North Sea

Abstract The changes in growth of plaice and sole between 1957 and 1988, as estimated from samples of the commercial fishery and pre-recruit surveys, were analysed in order to study possible density dependent effects. Indices of potentially competitive biomasses of plaice and sole, based on Lloyds index of mean crowding, were estimated from the average spatial distribution of various age groups during the summer growing period and from the population age structure as estimated by virtual population analysis. Growth of all age groups of sole increased in the 1960s and was stable in the 1970s and 1980s. In plaice only age groups 1 to 3 showed a similar increase in the 1960s, whereas the growth of 1-year-old plaice tended to decrease in the 1980s. Growth did not show a negative correlation with mean crowding, except in age group 1 of plaice and in age group 3+ of sole. It is concluded that these negative correlations do not provide unequivocal evidence for density dependent growth in plaice and sole, since they could equally well be caused by parallel but unrelated trends in time of one or more other factors. The simultaneous increase in growth in the 1960s of age groups of sole and plaice in the southern North Sea, and the absence of such an increase in age groups in the central North Sea, suggests that food availability must have increased in the Southern North Sea. This inference is supported by several macrobenthos studies. Whether the reduced growth of 1-group plaice in the 1980s, when recruitment was well above the average level, is caused by density dependent growth or to a reduced food availability remains an open question.