H.W. van der Veer

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.

Recruitment variability in dab (Limanda limanda) in the southeastern North Sea

Data from three annual surveys, covering inshore and offshore waters of the southeastern North Sea, were analysed to study recruitment variability in dab (Limanda limanda) over the period 1978-1997. Geometric mean abundance of 0- to 5-group dab was estimated using general linear models. Juvenile dab (0- and 1-group) were found over the entire area, from inside the estuaries to 50 m depth offshore. Environmental conditions (water temperature, wind stress, turbidity) affected the catch rates. The potential errors in the estimates of year-class strength, caused by differences in catchability, are discussed. The inter-annual pattern of year-class strength appeared to be established between ages 1 and 2, suggesting that factors determining recruitment are not restricted to the pelagic early life phase only, but also operate during the demersal juvenile phase. Recruitment variability at age 2 was in the order of 50-60nd appears to be equal to, or lower than, recruitment variability in plaice and sole. These results contradict expectations based on the concentration hypothesis, which states that the degree of variation in recruitment is inversely related to the degree of concentration during early life phases

The suitability of a single intertidal fish trap for the assessment of long-term trends in fish and epibenthic invertebrate populations

The suitability of a single intertidal fish trap for the assessment of long-term trends in fish and epibenthic invertebrate relies upon the assumption of an on-average constant sampling fraction from the study population. This assumption was examined by comparing trends over years in five different traps within the study area. For some species, like cod, the traps showed more or less parallel fluctuations. This is in agreement with the assumption. However, for other species, like mackerel, the assumption was invalidated by distinct trends over years.

The European sea bass Dicentrarchus labrax in the Dutch Wadden Sea: from visitor to resident species

This paper analyses the population dynamics, growth and feeding ecology of Dicentrarchus labrax in order to gain a better understanding of its present role in the western Dutch Wadden Sea ecosystem. Otolith analysis showed that the population is mostly comprised of individuals aged 3–5 years old and between 20 and 45 cm in length. In autumn, 0-group juveniles are also an important part of the population. Both juveniles and adults use the area as a feeding ground exhibiting an opportunistic feeding strategy that relies on available prey, especially the brown shrimp Crangon crangon. Stomach content analysis and nitrogen stable isotope analysis showed an ontogenetic shift towards piscivory and a general decrease in the dominance of invertebrates with increasing size. Over the last 50 years, large between-year fluctuations in D. labrax abundance have been observed with an underlying increasing trend from about 1990 until 2007 followed by a subsequent decline. Spring abundance showed significant relationships with temperature and salinity while autumn abundance was only related to temperature. Spring and autumn D. labrax abundance were also strongly related to abundance of brown shrimp C. crangon prey. Long-term trends in temperature and salinity in the area suggest that environmental conditions for juvenile growth have become optimal, resulting in increased abundance since the mid-1980s. Continued monitoring of the dynamics of this species in the Dutch Wadden Sea is important to understand and anticipate the effects of climate change on the D. labrax population and its role in the local food web

Movement, connectivity and population structure of the intertidal fish Lipophrys pholis as revealed by otolith oxygen and carbon stable

ABSTRACT The shanny Lipophrys pholis is an intertidal fish commonly found in Portuguese coastal waters. Spawning takes place from early autumn to mid spring, after which demersal eggs hatch and larvae disperse along the coast. Two to three months later, young juveniles return to the tide pools to settle. However, information on fish movement, habitat connectivity and population structure is scarce for this species. One hundred and twenty early juveniles (16–35 mm) were collected in April 2014 from six rocky beaches along the western and south Portuguese coasts (Agudela, Cabo do Mundo, Boa Nova, Peniche, Sines and Olhos de Água). δ18O and δ13C were determined by isotope-ratio mass spectrometry. Data were analysed to determine whether isotopic signatures could be used to assess the degree of separation between individuals collected from different locations. Mean δ13C and δ18O values ranged from −0.02‰ to 1.14‰ and −7.77‰ to −6.62‰, respectively. Both seawater temperature and salinity caused differences in otolith δ18O among the four main sampling areas. The variation among areas in δ13C was most likely related to slight differences in the diet, growth and metabolism of fish. The distinct isotopic signatures, at least for the northern and central areas, suggested low levels of connectivity across large spatial scales during the juvenile stage. Furthermore, similar isotopic signatures within the same area indicated some degree of larval oceanic retention at short spatial scales. This study suggests that stable isotope records in otoliths could provide information about the home residency, movements and habitat connectivity of intertidal fishes.

Spatial and temporal trends in order richness of marine phytoplankton as a tracer for the exchange zone between coastal and open waters

Quantifying exchange of particulate matter between coastal and open waters is an important and often unresolved issue. Here, we apply phytoplankton order richness as an innovative marine tracer to identify the geographic position of a coastal exchange zone in the SE North Sea, including its variability in time and space. Previous observations on dynamics of suspended particulate matter accumulation resulted in a hypothesized boundary between coastal waters (including the Wadden Sea) and open North Sea waters, the so-called ‘line-of-no-return’. Our study along two transects (Terschelling, Noordwijk) in the Dutch coastal zone showed seasonality patterns in phytoplankton order richness, both for diatoms and flagellates. The coastal Wadden Sea was found to be clearly different from the open North Sea, implying that seasonality in Wadden Sea phytoplankton is at least partly driven by local environmental conditions. Seasonality in flagellates was found to be more uniform than seasonality in diatoms. Stations in the coastal North Sea to a distance of 10 km (Terschelling) to 20 km (Noordwijk) from the shore appeared to be at the inside of the ‘line-of-no-return’. Our findings indicate that this approach is a useful aid in exploring mixing of particulate matter between coastal and open waters and to study the responses of phytoplankton communities to environmental drivers.

The 1996 Flatfish Symposium; a summary report

Abstract A summary report of the 1996 Flatfish Symposium is presented. Three topics of the Symposium are discussed: taxonomy and systematics, recruitment processes, and management issues. It is strongly recommended that a taxonomic advisory committee should be formed that will facilitate the transfer of information between different scientific disciplines focused on increasing our knowledge about flatfishes. Recruitment variation in flatfish appears to be dominated by density-independent factors operating at a local scale on the eggs and larvae. Variability in hydrodynamic circulation is suggested to be an important factor. Density-dependent processes are likely to be restricted to the smaller size classes which are not affected by fishing. The debate on the species range hypothesis is continuing. The patterns in latitudinal variation observed in fish did not correspond with the expectations. However, the species range hypothesis could neither be accepted nor rejected, because other sources of variation such as water temperature, sampling strategy, age composition of catches, inshore–offshore gradients and shifts in species ranges are biasing and maybe overruling the patterns in recruitment. Evaluations of the management of different flatfish stocks stressed the importance of the interactions between management measures and flatfish population structures. This field should be strengthened in the future.