Mitigating seafloor disturbance of bottom trawl fisheries for North Sea sole Solea solea by replacing mechanical with electrical stimulation.

Ecosystem effects of bottom trawl fisheries are of major concern. Although it is prohibited to catch fish using electricity in European Union waters, a number of beam trawlers obtained a derogation and switched to pulse trawling to explore the potential to reduce impacts. Here we analyse whether using electrical rather than mechanical stimulation results in an overall reduction in physical disturbance of the seafloor in the beam-trawl fishery for sole Solea solea. We extend and apply a recently developed assessment framework to the Dutch beam-trawl fleet and show that the switch to pulse trawling substantially reduced benthic impacts when exploiting the total allowable catch of sole in the North Sea. Using Vessel Monitoring by Satellite and logbook data from 2009 to 2017, we estimate that the trawling footprint decreased by 23%, the precautionary impact indicator of the benthic community decreased by 39%, the impact on median longevity of the benthic community decreased by 20%, the impact on benthic biomass decreased by 61%, and the amount of sediment mobilised decreased by 39%. The decrease in impact is due to the replacement of tickler chains by electrode arrays, a lower towing speed and higher catch efficiency for sole. The effort and benthic physical disturbance of the beam-trawl fishery targeting plaice Pleuronectes platessa in the central North Sea increased with the recovery of the plaice stock. Our study illustrates the utility of a standardized methodological framework to assess the differences in time trends and physical disturbance between gears.

Differences in biological traits composition of benthic assemblages between unimpacted habitats.

There is an implicit requirement under contemporary policy drivers to understand the characteristics of benthic communities under anthropogenically-unimpacted scenarios. We used a trait-based approach on a large dataset from across the European shelf to determine how functional characteristics of unimpacted benthic assemblages vary between different sedimentary habitats. Assemblages in deep, muddy environments unaffected by anthropogenic disturbance show increased proportions of downward conveyors and surface deposit-feeders, while burrowing, diffusive mixing, scavenging and predation traits assume greater numerical proportions in shallower habitats. Deep, coarser sediments are numerically more dominated by sessile, upward conveyors and suspension feeders. In contrast, unimpacted assemblages of coarse sediments in shallower regions are proportionally dominated by the diffusive mixers, burrowers, scavengers and predators. Finally, assemblages of gravelly sediments exhibit a relatively greater numerical dominance of non-bioturbators and asexual reproducers. These findings may be used to form the basis of ranking habitats along a functional sensitivity gradient.

Temporal genetic stability and high effective population size despite fisheries-induced life-history trait evolution in the North Sea sole.

Heavy fishing and other anthropogenic influences can have profound impact on a species' resilience to harvesting. Besides the decrease in the census and effective population size, strong declines in mature adults and recruiting individuals may lead to almost irreversible genetic changes in life-history traits. Here, we investigated the evolution of genetic diversity and effective population size in the heavily exploited sole (Solea solea), through the analysis of historical DNA from a collection of 1379 sole otoliths dating back from 1957. Despite documented shifts in life-history traits, neutral genetic diversity inferred from 11 microsatellite markers showed a remarkable stability over a period of 50 years of heavy fishing. Using simulations and corrections for fisheries induced demographic variation, both single-sample estimates and temporal estimates of effective population size (N(e) ) were always higher than 1000, suggesting that despite the severe census size decrease over a 50-year period of harvesting, genetic drift is probably not strong enough to significantly decrease the neutral diversity of this species in the North Sea. However, the inferred ratio of effective population size to the census size (N(e) /N(c) ) appears very small (10(-5) ), suggesting that overall only a low proportion of adults contribute to the next generation. The high N(e) level together with the low N(e) /N(c) ratio is probably caused by a combination of an equalized reproductive output of younger cohorts, a decrease in generation time and a large variance in reproductive success typical for marine species. Because strong evolutionary changes in age and size at first maturation have been observed for sole, changes in adaptive genetic variation should be further monitored to detect the evolutionary consequences of human-induced selection.

Temporal changes in allele frequencies but stable genetic diversity over the past 40 years in the Irish Sea population of thornback ray, Raja clavata.

Rays and skates are an unavoidable part of the by-catch in demersal fisheries. Over the past 40 years, the thornback ray (Raja clavata) has decreased in numbers and even disappeared in some areas, leading to concerns about genetic risk. For this reason, the effective population size (N(e)), the migration rate (m) and temporal changes in the genetic diversity were estimated for the population of thornback rays in the Irish Sea and Bristol Channel. Using genotyped, archived and contemporary samples (1965 and 2003-2004), N(e) was estimated at 283 individuals (95% CI=145-857), m at 0.1 (95% CI=0.03-0.25) and the N(e)/N ratio between 9 x 10(-5) and 6 x 10(-4). Although these results must be treated with caution, due to the small sample sizes, this is the first attempt to estimate N(e) in an elasmobranch species. The low N(e)/N ratio suggests that relatively few individuals contribute to the next generation. The combined effect of sex bias, inbreeding, fluctuations in population size and, perhaps most important, the variance in reproductive success may explain the low N(e)/N ratio. In addition, the relatively high gene flow between Irish Sea population and other source populations is likely to have had an impact on our estimate, which may be more relevant at the metapopulation scale. No significant loss of genetic diversity was found over the 40-year timeframe and long-term maintenance of the genetic diversity could be due to gene flow.

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 H(exp)= 0.744 +/- 0.02). Pairwise comparisons of theta; among all 11 locations revealed significant differentiation between Iceland and all other locations (theta = 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.

Fisheries as a large-scale experiment on life-history evolution: disentangling phenotypic and genetic effects in changes in maturation and reproduction of North Sea plaice, Pleuronectes platessa L.

This paper attempts to interpret the observed changes in reproductive strategy of female North Sea plaice since 1900 in the light of possible genetical selection exerted by the fisheries. Somatic growth of juvenile plaice increased between the 1950s and the 1980s, probably as a response to an increase in food availability. Adult growth rate was constant, except during a period of increased population abundance when somatic growth decreased. Both length (L mat) and age at first sexual maturity decreased since 1990. No firm evidence was obtained for a change in total reproductive investment, although size-specific fecundity was reduced in the period of increased population abundance, suggesting a trade-off between egg numbers and egg size. Analysis of the phenotypic response of maturation to an increase in juvenile growth suggested that only a part of the decrease in L mat could be ascribed to the observed increase in juvenile growth. The unexplained part of the change in L mat corresponded with the predicted change due to genetical selection by the fisheries. This supported the hypothesis that fishing caused a genetical change in L mat, although an unequivocal interpretation is not possible from a descriptive study.

Pattern of movement in and dispersal from a dutch forest of Carabus problematicus Hbst. (Coleoptera, Carabidae).

Data in the literature that in the Netherlands Carabus problematicus is an autumn breeder with overwintering larvae and adults have been confirmed. Pitfall catches show two peaks of adult beetles. The first small peak of activity is realized in May, the significance of which is not yet understood. The second higher peak occurs from the end of August till the beginning of October: the reproduction period. In this period at least three age classes are present. Some females were shown to take part in reproduction for at least two seasons. Old females appeared to have less eggs in the ovaries than young ones. Larvae can be trapped throughout the winter. The new generation appears in July and in the beginning of August. They immediately take part in reproduction. C. problematicus shows noctural activity with a peak towards the end of the night.In field experiments the pattern of movement and the dispersal behaviour of marked individuals was investigated and related to the choice of habitat. It was shown that in the Netherlands C. problematicus prefers a forest habitat. Some individuals were found to disperse from their favoured habitat. The percentage of beetles that disperse out of the forest during the breeding season could be estimated at 4-8%. Dispersal increases in the second part of the breeding season. Females tend to disperse more frequently than males.The pattern of movement consists of two types: a "random walk" and a "directed movement". It is suggested that the pattern of movement of individuals dispersing out of the forest is the directed movement. It could be shown that individuals released outside the forest were able to walk towards a forest silhouette. The significance of both the pattern of movement and the dispersal behaviour for the survival of the species is discuussed.