Loes J. Bolle

Nursery grounds of dab (Limanda limanda L.) in the southern North Sea

Abstract The distribution of 0-group dab, Limanda limanda L., in the open North Sea, the Dutch coastal zone and the estuarine Wadden Sea was investigated and patterns of seasonal abundance and growth were compared. The purpose was to locate the nursery grounds of dab and to assess their relative importance. Settlement was observed in the North Sea, both in coastal and offshore waters. Highest densities of 0-group dab were found in coastal areas. Since the coastal zone covers only a relatively small part of the North Sea it is not necessarily the most important nursery area for dab. No settlement of dab occurred in the Wadden Sea. Large numbers of juveniles were caught in the tidal channels and subtidal areas in autumn, but their size range indicated that migration rather than settlement was responsible for the observed increase in numbers. A similar process of inshore migration occurred in shallow coastal waters in October. Growth of 0-group dab was more or less similar in all areas. Mean length increased to 6–7 cm by the end of the first year of life.

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

Influence of DNA isolation from historical otoliths on nuclear–mitochondrial marker amplification and age determination in an overexploited fish, the common sole (Solea solea L.)

Historical otolith collections are crucial in assessing the evolutionary consequences of natural and anthropogenic changes on the demography and connectivity of commercially important fish species. Hence, it is important to define optimal protocols for purifying DNA from such valuable information sources while avoiding any damage to the physical structure of the otolith. Before being able to conclude on the harmlessness of a method, it is important to validate protocols on different kinds of otoliths by testing purification methodologies under standardized conditions. Here we compare the effect of two DNA extraction methods on the success in identifying the age in an overexploited marine fish, the common sole (Solea solea L.). To ensure optimal future population genetic and demographic analyses, we assessed DNA quantity and tested the DNA quality by investigating the amplification success of a mitochondrial and nuclear marker. Our results show that the choice of the DNA extraction method had a significant effect on the success of using these otoliths in age and growth analyses. Standard commercial and published protocols resulted in a severe damaging of the otolith structure, hampering accurate preparation and analyses of the morphological structures of the otoliths. Shortening the lysis time and lowering the EDTA (ethylene diamine tetraacetic acid) and SDS (sodium dodecylsulphate) concentration turned out to be beneficial for the stability of otolith structure, while maintaining an overall high DNA quality measured through polymerase chain reaction amplification success. We therefore recommend that care should be taken when choosing the extraction method for a molecular study on archived samples, in order to enable the maximal use of information embedded in historical material.

Ecological observations on juvenile flatfish in a tropical estuary: Arquipélago dos bijagós, Guinea-Bissau

Abstract Juvenile flatfish were studied in a tropical estuary, Arquipelago dos Bijagos in Guinea-Bissau from October 1992 until April 1993. A total of eight flatfish species, belonging to the families Bothidae, Cynoglossidae, Psettodidae and Soleidae, were found. In comparison with other fish groups, flatfishes abundance was low with densities of less than 13 ind·100 m −2 . One species, Citharichthys stampflii , occurred in densities high enough to permit an analysis of distribution, growth and mortality. During the period of study, settlement of C. stampflii was observed in the estuary. Its distribution suggested a preference for offshore areas, densities decreasing towards the river systems. Repeated sampling in the intertidal area of Ilheu de Flamingos permitted a preliminary estimate of mortality and growth. Instantaneous mortality rates were about 0.03·d −1 , which is similar to estimates in temperate estuaries. Growth rates were slightly above 1 mm·d −1 , about the same as those measured in subtropical estuaries.

Ecologisch Gericht Suppleren: meetplan geïntegreerde ecosysteem survey 2017

Suppleties van zand op vooroever of strand worden in opdracht van Rijkswaterstaat uitgevoerd om de Nederlandse kust tegen erosie te beschermen en om voldoende zand in het kustfundament te houden. Een groot deel van de suppleties vindt plaats in of nabij de kuststrook die binnen de Natura2000 regelgeving wordt beschermd, de Noordzeekustzone. Het is dus van belang de eventuele effecten van deze praktijk op de natuur zorgvuldig te bestuderen, zodat dit effect kan worden afgezet tegenover het algemene nut voor de maatschappij. Betere kennis van de effecten kan leiden tot beperking van eventuele schade aan- en mogelijk zelfs tot versterking van- gewenste natuurwaarden en ecosysteemdiensten. Tot nog toe is er relatief weinig aandacht geweest voor de gevolgen van suppleren op vispopulaties in vergelijking met benthos, terwijl de kinderkamerfunctie van de ondiepe kustzone een zeer belangrijke economische ecosysteemdienst levert. Kennis van de habitatfactoren die het voorkomen van juveniele vis in kinderkamers bepalen leidt tot een verbeterd inzicht van de gevolgen van suppleties op vispopulaties en van de voedselketen van viseters in de ondiepe kustzone. In overleg met natuurorganisaties en de kennisinstituten Deltares en Wageningen Marine Research is in 2016 het document `Ecologische effecten van zandsuppleties’ (Herman et al., 2016) geschreven met als doel onderzoek te formuleren naar ecologische effecten van zandsuppleties. In het onderdeel ‘uitvoeringsplan’ (deel C in Herman et al. 2016) zijn 3 onderzoekslijnen (ook wel Krachtlijnen genoemd) gedefinieerd, te weten: Vooroever, Duinen en Waddenzee. Het hier beschreven meetplan voor een survey in 2017 valt onder de onderzoekslijn Vooroever. De onderzoeksvraag die in dit meetplan wordt behandeld volgt uit de prioritering van de krachtlijn Vooroever: (Cumulatieve) gevolgen van reguliere suppleties op samenstelling en functioneren van het ecosysteem van de vooroever. Deze onderzoeksvraag luidt: “Wat zijn de cumulatieve gevolgen van reguliere suppleties op samenstelling en functioneren van het ecosysteem van de ondiepe vooroever van de Nederlandse kust?” Conform het plan van aanpak voor dit programma (Herman et al., 2016) wordt voorgesteld om een survey uit te voeren in de vooroever (0 tot 10-12 m diepte), waarbij benthos, vis en habitatkarakteristieken worden bemonsterd. Deze geintegreerde ecosysteem survey is daarmee een onderdeel van een groter pakket van geplande dataverzameling in het kader van Ecologisch Gericht Suppleren II. Het hoofddoel van de survey is om data te verzamelen over het voorkomen van (juveniele) vis in relatie tot relevante omgevingsvariabelen, zowel abiotisch als biotisch. De survey zal inzicht verschaffen in het functioneren van het kustecosysteem en kennis opdoen over de wisselwerking met biotische en abiotische omgevingsvariabelen. De resultaten van de survey zullen dienen voor het opstellen van habitatmodellen voor juveniele vis om hiermee effecten van suppleties te kwantificeren. Dit rapport beschrijft de meetstrategie, de meetmethoden, de te meten variabelen (vis, benthos en omgevingsvariabelen) en de bemonsteringslocaties voor de ondiepe geintegreerde ecosysteem survey 2017.