Henk W. van der Veer

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.

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.

Importance of predation by crustaceans upon bivalve spat in the intertidal zone of the Dutch Wadden Sea as revealed by immunological assays of gut contents

Predation on bivalve spat (Macoma balthica, Cerastoderma edule and Mya arenaria) by crustaceans was studied in an intertidal area in the western Dutch Wadden Sea in 1993. In all species settlement started at the end of April. Peak numbers were reached in the beginning of June of about 4300 ind. m−2 in Macoma, 1300 in Cerastoderma and 500 in Mya. During June and July numbers dropped sharply to low levels at a mean size of 1.0 to 1.5 mm. Instantaneous mortality rates over this period varied from 0.045 d−1 in Macoma to 0.056 d−1 in both Cerastoderma and Mya. Immunoassays of gut contents of brown shrimps Crangon crangon and shore crabs Carcinus maenas positively identified both species as predators on bivalve spat. The high shrimp densities observed (100 ind. m−2) in combination with their simultaneous presence with the post-larval bivalves indicate that juvenile shrimps are likely the most important predator. The estimated predation pressure by crustaceans, and especially the brown shrimp, is sufficient to account for the observed mortality rates among bivalves. Bivalve recruitment, however, did not appear to be regulated by crustacean predation: during the period of predation, bivalve mortality was not density-dependent. Information on bivalve predation in Swedish bays support this view.

Temperature tolerance and energetics: a dynamic energy budget-based comparison of North Atlantic marine species

Temperature tolerance and sensitivity were examined for some North Atlantic marine species and linked to their energetics in terms of species-specific parameters described by dynamic energy budget (DEB) theory. There was a general lack of basic information on temperature tolerance and sensitivity for many species. Available data indicated that the ranges in tolerable temperatures were positively related to optimal growth temperatures. However, no clear relationships with temperature sensitivity were established and no clear differences between pelagic and demersal species were observed. The analysis was complicated by the fact that for pelagic species, experimental data were completely absent and even for well-studied species, information was incomplete and sometimes contradictory. Nevertheless, differences in life-history strategies were clearly reflected in parameter differences between related species. Two approaches were used in the estimation of DEB parameters: one based on the assumption that reserve hardly contributes to physical volume; the other does not make this assumption, but relies on body-size scaling relationships, using parameter values of a generalized animal as pseudo-data. Temperature tolerance and sensitivity seemed to be linked with the energetics of a species. In terms of growth, relatively high temperature optima, sensitivity and/or tolerance were related to lower relative assimilation rates as well as lower maintenance costs. Making the step from limited observations to underlying mechanisms is complicated and extrapolations should be carefully interpreted. Special attention should be devoted to the estimation of parameters using body-size scaling relationships predicted by the DEB theory.

Importance of exposure and habitat structure for the population density of 0-group plaice, Pleuronectes platessa L., in coastal nursery areas

Density of 0-group plaice (Pleuronectes platessa L.) during peak abundance in spring (June) was recorded in 22 coastal nursery areas on the Swedish west coast. Relationships between plaice density and some local environmental factors as exposure, habitat structure, food availability and predation pressure in the nursery areas were studied, and the importance of these factors was evaluated. Exposure affects habitat structure (sediment and vegetation) which might influence settling behaviour and survival of juvenile plaice. Highest plaice densities were observed in semi-exposed bays. Shortly after settling of plaice larvae, predation by crustaceans will be a key factor controlling population density of juvenile plaice, as high predation densities went together with low plaice densities. Food availability in the nursery areas was not found to be related to plaice density.

Impact of interannual variability in hydrodynamic circulation on egg and larval transport of plaice Pleuronectes platessa L. in the southern North Sea

Abstract A realistic 2D circulation model of the southern North Sea was applied to simulate the interannual variability in dispersal (advection and diffusion) of plaice Pleuronectes platessa L. eggs and larvae from the spawning area in the Southern Bight of the North Sea towards the Dutch coastal nursery areas. The model is driven with daily varying current fields, which are the result of driving a general hydrodynamical model with realistic tidal and meteorological (wind) forcing for the period 1974–1981. Model simulations of plaice egg and larval transport are compared with observed larval concentrations near coastal nursery areas (Delta area and Wadden Sea). The model simulations show that: (1) the interannual variability in transport is large and of the same order as the year-to-year variations observed in larval concentrations near the nursery areas; (2) the interannual variability increases with increasing distance from the spawning area; (3) for the years in which larval estimates were available, the model predictions were positively correlated with observed estimates for the Marsdiep tidal inlet. This suggests that the variability in circulation patterns during the early pelagic stages in the open sea might be a key factor in determining year-class strength of plaice.

Variations in settlement and growth of 0-group plaice (Pleuronectes platessa L.) in the dutch wadden sea as determined by otolith microstructure analysis

Abstract Larval growth, date of settlement, and subsequent growth of 0-group plaice were compared in the eastern (Dollard) and western (Texel) parts of the Dutch Wadden Sea in autumn 1988. Daily increment analysis in otoliths with SEM showed that specimens sampled in the Texel region, and at 2 Dollard stations, hatched early in the season, had a shorter larval period (50 to 61 d) with high growth rates, and early settlement. All specimens caught at the other 6 stations hatched later, had a long larval phase (62 to 82 d) and settled later in the season. Estimates of maximum growth based on optimal feeding conditions and the prevailing water temperatures in both tidal basins were compared to the growth rates observed in the field. According to previous studies, plaice in the Texel region have optimal growth. All late settlers in the Dollard and part of the early settlers also had optimal growth. However, some early settlers in the Dollard had significantly lower growth rates than the batch of synchronously settled specimens from the Texel region.

Phylogeography of the common shrimp, Crangon crangon (L.) across its distribution range

The common or brown shrimp Crangon crangon (L.) is a highly abundant and important taxon, both ecologically and commercially, yet knowledge on its population structure and historical biogeography is limited. We studied population genetic structure across the distribution range of this species by sequencing a 388 bp fragment of the cytochrome-c-oxidase I gene for 140 individuals from 25 locations. Strong population structuring and high levels of genetic diversity were observed. Four main phylogroups were uncovered: northeastern Atlantic, western Mediterranean, Adriatic Sea and Black Sea. Gene flow of these shrimp across known oceanographical barriers (e.g., the Strait of Gibraltar and/or Oran-Almeria front, Sicilian Straits, and Turkish Straits) is severely restricted. The oldest and most variable populations currently inhabit the western Mediterranean. The observed absence of structure across the entire northeastern Atlantic shelf is proposed not to be due to gene flow, but to relatively recent colonization following the glacial cycles of the late Pleistocene. Black Sea shrimp are currently disconnected from Mediterranean populations, and colonization is inferred, on the basis of coalescent analysis, to have happened relatively recently, but possibly earlier than 7000 years ago. We postulate the hypothesis that C. crangon survived the last brackish-water (<7 per thousand) period inside the Black Sea and/or one of the adjacent inland seas. We conclude that (1) common shrimp populations from different basins are strongly differentiated, (2) gene flow across basins is probably very limited, and (3) the biogeographic history of the taxon is largely in accordance with the geographic history of its distribution range. This study provides further evidence that high population connectivity of marine species (e.g., by policy makers) should not be assumed.

Competition in deposit- and suspension-feeding bivalves: experiments in controlled outdoor environments

Experiments were carried out in an intertidal and a subtidal outdoor flow-through basin. The effect of bivalve density on growth and survival was studied in the facultative deposit feeder Macoma balthica (L.) and the obligate suspension feeder Cerastoderma edule L. Patches of 0.2 m2 with densities of maximal 2480 bivalves per m2 were used. In C. edule growth was density independent, whereas in M. balthica growth decreased with density, indicating intraspecicic competition in M. balthica, but not in C. edule. These different feeding in C. edule. In mixed populations, C. edule and M. balthica did nopt clearly influence each others growth and survival, indicating that interspecific competition did not occur between the two species. Our experiments support the hypothesis of levinton (1972), that, on local scale, competition is expected more readily in deposit-feeding communities than in suspension-feeding communities.

Major changes in the ecology of the Wadden Sea: Human impacts, ecosystem engineering and sediment dynamics

Shallow soft-sediment systems are mostly dominated by species that, by strongly affecting sediment dynamics, modify their local environment. Such ecosystem engineering species can have either sediment-stabilizing or sediment-destabilizing effects on tidal flats. They interplay with abiotic forcing conditions (wind, tide, nutrient inputs) in driving the community structure and generating spatial heterogeneity, determining the composition of different communities of associated species, and thereby affecting the channelling of energy through different compartments in the food web. This suggests that, depending on local species composition, tidal flats may have conspicuously different geomorphology and biological functions under similar external conditions. Here we use a historical reconstruction of benthic production in the Wadden Sea to construct a framework for the relationships between human impacts, ecosystem engineering and sediment dynamics. We propose that increased sediment disturbances by human exploitation interfere with biological controls of sediment dynamics, and thereby have shifted the dominant compartments of both primary and secondary production in the Wadden Sea, transforming the intertidal from an internally regulated and spatially heterogeneous, to an externally regulated and spatially homogenous system. This framework contributes to the general understanding of the interaction between biological and environmental control of ecosystem functioning, and suggests a general framework for predicting effects of human impacts on soft-bottom ecosystems.