Johannes Ij. Witte

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.

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.

Small trawls in juvenile flatfish research: Their development and efficiency

A resume is given of the sampling techniques applied in juvenile flatfish research in the countries around the North Sea in the course of time with the emphasis on small trawl nets. A preliminary comparison of the relative efficiencies of otter-trawl and beam-trawl under different environmental conditions is presented. Beam-trawls show a higher and less variable efficiency than otter-trawls. The fixed width of a beam-trawl may be an important factor. Based on these results, a standardization of juvenile flatfish sampling techniques is suggested, based on the 1-m and 2-m beam-trawl.

On the ecology of the dragonet Callionymus lyra L. in the southern North Sea

Abstract The present paper evaluates data collected in the southern North Sea between 1972 and 1984 on geographical distribution, density, growth, mortality, biomass, production and food requirements of the dragonet Callionymus lyra L. Catches never exceeded 1500 individuals per 10 000 m2 and the dragonet showed a clear pattern of abundance with highest densities in the coastal zone and decreasing numbers going offshore. Mortality rates appeared to be rather constant over the age groups with z = 0.55 y−1. The dragonet population mainly consisted of 1, 2 and 3-year-old individuals with a mean length at the end of the year of 7.5, 15 and 19 cm, respectively. Production values were at least 159 g AFDW·10 000 m−2·y−1. Predation pressure exerted by the dragonets amounted to at least 795 g AFDW·10 000 m−2·y−1. The correspondence between the von Bertalanffy growth curve in this study and that of 1948 ( Chang , 1951) suggests absence of effects of eutrophication in the area on the growth of dragonets between 1948 and 1980s. The main difference with 1948 was the absence of large individuals, probably as a result of increased fishery in the area.

Intertidal fish traps as a tool to study long-term trends in juvenile flatfish populations☆

Abstract The suitability of intertidal fish traps (fykes, komfykes and koms) to study long-term trends in juvenile flatfish (plaice, flounder, dab and sole) was analysed with respect to effects of location, type of trap and fishing duration. For all species, catches by the three different types of fykes and on the three locations studied differed considerably, but the patterns over time were approximately the same. The results further suggest that komfykes have the highest catches of flatfish and koms the lowest. The similarity in fluctuations of catches between fish traps and between locations suggest that they may be suitable for the study of long-term changes in flatfish populations. For all species, increasing fishing duration from 24 to even up to 96 h hardly resulted in higher catches.

Morphological differences in Macoma balthica (Bivalvia, Tellinacea) from a Dutch and three southeastern United States estuaries

Abstract Field collections of the bivalve Macoma balthica in the Dutch Wadden Sea and three southeastern United States estuaries revealed morphological differences between populations of the two sides of the Atlantic Ocean. Individuals of the same age showed much larger shell lengths at the American stations. In addition, bivalves of the same body weight had higher siphon weights at the American stations than at the Dutch stations. This difference in siphon size was related to their burying depths. The American population, which invested more in heavier siphons, was able to burrow much deeper into the sediment (up to 30 cm). Deep burial may be an adaptation to avoid exposure to the high southern temperatures. Furthermore, it may serve as a refuge from blue crab predation. The results of our comparison between the southern American population and the European support the suggestion that populations of M. balthica living in these two regions should be considered separate and sibling species.

Is reproductive failure responsible for reduced recruitment of intertidalMytilus edulis L. in the western Dutch Wadden Sea

The musselMytilus edulis is an abundant bivalve in the Dutch Wadden Sea, both in intertidal areas as well as in subtidal culture plots. From mid 1980’s to late 1990’s, strong declines in mussel intertidal populations have been observed, both in terms of occupied area and in biomass. Despite the efforts since 1999 to preserve intertidal mussel beds, abundance and biomass of mussels in the Dutch Wadden Sea are at present still much lower than in the 1970’1s. In the present paper, we tested whether reproductive failure could have been the cause for the low recruitment ofM. edulis in the western Dutch Wadden Sea. Water temperature is an important factor influencing growth and reproduction in bivalves. The observed increase in mean temperature in the Dutch Wadden Sea during the last decades suggests that an effect of temperature on the population dynamics of mussels might be expected. Nevertheless, no significant relationship was found between body condition and mean winter temperature. Therefore, a decrease in body condition and, consequently, in reproductive output, due to increasing temperature, will not have been a cause for the observed low recruitment in recent years. The seasonal patterns of individual growth supported this view: most adults developed gonads during the spawning season suggesting that reproductive (gametogenic) failure was not a cause for the low recruitment. Nevertheless, reproductive investment by the mussel population studied was not optimal since complete spawning hardly occurred. In addition, the high gonadosomatic ratio at the end of the summer did not result in heavy spawning, suggesting resorption of gonadal tissue due to unfavourable environmental conditions (too high temperature and/or food limitation during summer). Reduced recruitment could be due to poor spawning but other factors that exert their influence after spawning (predation on larvae or postlarvae, lack of settlement substrate) or on the adult population (decrease in number of spawners) are also likely to be involved in the observed low recruitment during the last years.

Possible loss of polychlorinated biphenyls from migrating European silver eels: a 3 month simulation experiment

Abstract A 3 month experiment was set up to simulate the polychlorinated biphenyl (PCB) congener behavior in lipids during migration of silver eels, Anguilla anguilla , from river to sea. The thesis is that there should be loss of PCB contamination, to establish a new chemical equilibrium between the lipid and water compartments, where metabolism is the vehicle. Male and female eels collected in the River Rhine were placed separately, and in duplicate, in aquaria with running seawater of 5°C under starvation conditions for 91 days. Eels were sampled at regular intervals for the determination of eel-conditional parameters and PCB congener analysis. Also, water and particulate matter were analyzed for PCB congeners. Wet weight, lipid content, condition of organs such as heart, liver and spleen did not change throughout the experiment, at least not within the accuracy of the investigations and there was no indication of sexual maturation. This agrees with literature information and, for loss of weight, with theoretical calculation based on energy budget. The PCB congener chromatograms of eels showed little difference in time, and did not demonstrate any loss from the lipid compartment due to exposure to less contaminated seawater. The eel chromatograms showed the typical congener distribution with higher values at 2/3 of the chromatogram, also found earlier for mussels, and relative to that of water. The process of attaining a new partitioning equilibrium between the lipid compartment in gill-breathing aquatic organisms and that of water was not found. Neither were there changes in the PCB congener chromatograms due to selective metabolic activities. Using PCB congener No. 138 as reference, a congener that remained rather constant in the lipids of eels, only two congeners, Nos. 105 and 118, showed a significant increase with respect to No. 138. Starvation possibly retards metabolic activity and thus also inhibits loss of PCB congeners to the seawater compartment. Therefore, PCBs may remain as toxic contaminants in the lipids of fasting and migrating eels, which potentially may have noxious effects during migration and maturation. However, a noxious effect was not found during this experiment.

Ecological observations on juvenile flatfish in a tropical coastal system, Puerto Rico

Abstract Juvenile flatfish were sampled in Puerto Rico in a number of different habitat types: river mouths, beach systems, estuarine lagoons and marine lagoons. Out of 16 species occurring around Puerto Rico, seven were found, of the family Bothidae: the bay whiff Citharichthys spilopterus , the sand whiff C. arenaceus , the fringed flounder Etropus crossotus and the channel flounder Syacium micrurum , of the family Solidae: the lined sole Achirus lineatus and the scrawled sole Trinectes inscriptus , and of the family Cynoglossidae: the blackcheek tonguefish Symphurus plagiusa . Species composition varied among areas and appeared to be related to habitat requirements. Depending on species, settlement was prolonged over a period from 1 to at least 6 months. Abundances of 0-group were low, in general less than 10 ind·100 m −2 . Preliminary growth estimates in Citharichthys spilopterus were within the range of those observed in subtropical waters at the same temperatures.

Inventory and comparison of abundance of parasitic copepods on fish hosts in the western Wadden Sea (North Sea) between 1968 and 2010

A conspicuous part of the parasite fauna of marine fish are ectoparasites, which attach mainly to the fins or gills. The abundant copepods have received much interest due to their negative effects on hosts. However, for many localities the copepod fauna of fish is still poorly known, and we know little about their temporal stability as long-term observations are largely absent. Our study provides the first inventory of ectoparasitic copepods on fish from the western Wadden Sea (North Sea) based on field data from 1968 and 2010 and additional unpublished notes. In total, 47 copepod parasite species have been recorded on 52 fish host species to date. For two copepod species parasitizing the European flounder (Platichthys flesus), a quantitative comparison of infection levels between 1968 and 2010 was possible. Whereas Acanthochondria cornuta did not show a change in the relationship between host size and infection levels, Lepeophtheirus pectoralis shifted towards the infection of smaller hosts, with higher infection levels in 2010 compared to 1968. These differences probably reflect the biology of the species and the observed decrease in abundance and size of flounders during the last decades. The skin-infecting L. pectoralis can probably compensate for dwindling host abundance by infecting smaller fish and increasing its abundance per given host size. In contrast, the gill cavity inhabiting A. cornuta probably faces a spatial constraint (fixed number of gill arches), thus limiting its abundance and setting a minimum for the host size necessary for infections.