Estuarine fish passes in the northern Netherlands provide contrasting windows of opportunity for migrating fish species

The Wadden Sea in north-western Europe is the largest system of connected intertidal sand and mud flats in the world and is included in the UNESCO World Heritage list. Notwithstanding its ecological value, almost all rivers flowing towards the Wadden Sea are heavily modified with inter-tidal barriers, hampering the migration of diadromous fishes. To mitigate the negative effects of such barriers, fish passes were constructed, but their effectiveness is poorly understood. Since most fish passes are only functional during part of the tidal cycle, migration activity should match this operation window to secure effective migration. In this study we focused on the upstream-migrating behaviour of glass eel (the juvenile stage of catadromous eel, Anguilla anguilla) and three-spined stickleback (anadromous Gasterosteus aculeatus) at nine fish passes along the Dutch Wadden Sea coast. We investigated the presence of sticklebacks and glass eel at the downstream side of intertidal barriers in relation to fluctuations in their environment. Fish were collected with 1x1-m lift nets, with a 2-mm mesh size. In 168 sample days between February and May 2014, 4558 samples were taken and a total of 107,349 sticklebacks and 23,082 glass eel were caught. At all locations, glass eel were most abundant in the hour before high tide, while sticklebacks did not show a significant pattern with environmental variables at all locations, but when they did catches were highest at the start of flood tide. This means that the window of opportunity for migration is different for both fish species and that optimisation of the operation window of a fish pass for glass eel could mean a mismatch for stickleback and vice versa. Knowledge of temporal migration dynamics of fish species is therefore essential for the design of effective fish passes

Growth, production and food preference of rohu Labeo rohita (H.)in monoculture and in polyculture with common carp Cyprinus carpio (L.) under fed and non-fed ponds

An experiment was carried out in 18 earthen ponds to investigate the effects of the addition of common carp Cyprinus carpio (L.) and artificial feed on natural food availability, food utilization and fish production in rohu Labeo rohita (Hamilton) ponds. Ponds were fertilized fortnightly with cow manure, urea and triple super phosphate. Rohu was stocked in all ponds at a density of 1.5 rohu m-2. All treatments were carried out in triplicate. Treatments were: rohu with and without formulated feed, rohu plus 0.5 common carp m-2 with and without feed, and rohu plus 1 common carp m-2 with and without feed. The time period between stocking and harvesting was four and half months. Stocking 0.5 common carp m-2 enhanced natural food availability in the pond, food utilization and rohu growth and production (P0.05) but increased zooplankton and benthic macroinvertebrate availability (P<0.001). Feed administration also enhanced growth of rohu and common carp (P<0.001). Rohu naturally ingests more phytoplankton than zooplankton but in the presence of formulated feed rohu shifted its natural food preference from phytoplankton to zooplankton. Common carp naturally ingests mainly zooplankton and benthic macroinvertebrate and small quantities of phytoplankton. However, when offered formulated feed, the latter becomes the preferred food item

Disentangling the complexity of tropical small-scale fisheries dynamics using supervised Self-Organizing Maps

Tropical small-scale fisheries are typical for providing complex multivariate data, due to their diversity in fishing techniques and highly diverse species composition. In this paper we used for the first time a supervised Self-Organizing Map (xyf-SOM), to recognize and understand the internal heterogeneity of a tropical marine small-scale fishery, using as model the fishery fleet of San Pedro port, Tabasco, Mexico. We used multivariate data from commercial logbooks, including the following four factors: fish species (47), gear types (bottom longline, vertical line+shark longline and vertical line), season (cold, warm), and inter-annual variation (2007–2012). The size of the xyf-SOM, a fundamental characteristic to improve its predictive quality, was optimized for the minimum distance between objects and the maximum prediction rate. The xyf-SOM successfully classified individual fishing trips in relation to the four factors included in the model. Prediction percentages were high (80–100%) for bottom longline and vertical line + shark longline, but lower prediction values were obtained for vertical line (51–74%) fishery. A confusion matrix indicated that classification errors occurred within the same fishing gear. Prediction rates were validated by generating confidence interval using bootstrap. The xyf-SOM showed that not all the fishing trips were targeting the most abundant species and the catch rates were not symmetrically distributed around the mean. Also, the species composition is not homogeneous among fishing trips. Despite the complexity of the data, the xyf-SOM proved to be an excellent tool to identify trends in complex scenarios, emphasizing the diverse and complex patterns that characterize tropical small scale-fishery fleets.</p

Spatial and seasonal variation in the phytoplankton community of Lake Victoria’s Mwanza Gulf, compared to northern parts of the lake

We investigated the phytoplankton species composition and abundance in two seasons in Mwanza Gulf, Lake Victoria (Tanzania). Phytoplankton was sampled and chlorophyll a content was measured in the dry and wet seasons of 2010–2011 at three stations, from the southern land-inward end of the Gulf towards the open lake. Cyanobacteria, mostly small colonial and filamentous species (e.g., Aphanocapsa spp., Planktolyngbya spp., Merismopedia spp.) dominated at each station (76–95 %), followed by Chlorophyta (5–21 %), whereas the contribution of Bacillariophyceae was small (0–6 %). Phytoplankton densities were generally higher in the rainy season and strongly increased going land-inward from the open lake. Low abundance of N-fixing phytoplankton species suggests that N-fixation was low. The chlorophyll a content in the mouth of the Gulf was low (mean values 4–6 µg/L) compared to values reported previously. Also, chlorophyll a values (means 11–14 µg/L) at land-inward stations of Mwanza Gulf were much lower than those in the northern gulfs (Napoleon Gulf, Murchison Bay and Nyanza Gulf). Between 2002 and 2009 the phytoplankton composition of Mwanza Gulf changed from a community mostly dominated by Bacillariophyceae into a community dominated by Cyanobacteria. In the open water of Lake Victoria, Bacillariophyceae and Cyanobacteria were both abundant. Cyanobacteria dominated both in the three northern gulfs and Mwanza Gulf, but all four showed substantial differences in species and genus compositions. Phytoplankton composition and abundance in Mwanza Gulf differs in many respects from the open water of Lake Victoria and its three northern gulfs

Swimming, grazing and social behaviour of rohu Labeo rohita (Hamilton) and common carp Cyprinus carpio (L.) in tanks under fed and non-fed conditions

Fish behaviour was monitored in 1-m2 tanks each stocked with three 67-g rohu (Labeo rohita). In addition, 80-g common carp (Cyprinus carpio) were stocked at 0, 1 or 2 fish per tank. All tanks were fertilized prior to stocking to stimulate natural food production. In addition, half of the tanks were fed a supplemental diet. Results confirmthe general view that rohumainly lives and feeds in thewater column, while common carp is a bottomfeeder. In the presence of commoncarp, rohu increased the time spend grazing on tank wall and bottom, and decreasing grazing time in the water column. Supplemental feeding had a similar, be it less pronounced effect. When both present, rohu and common carp spend 47–52% of their time together. Rohu spends more time close to the bottomin the presence of common carp thanwhen no common carp is present, presumably to profit from increased zooplankton production, triggered by the resuspension of nutrients by burrowing common carp. This was reflected in a higher growth rate of rohu in the presence of carp. These effects were stronger with one commoncarp per tank thanwith two commoncarp. In this study, the results frombehavioural observations in tanks nicely complemented results from a pond study analyzing growth, production and food availability. Behavioural observations in tank yielded useful additional information helpful to clarify species interactions and feeding ecology in polyculture ponds

Relationships among water quality, food resources, fish diet and fish growth in polyculture ponds: a multivariate approach

We examined the influence of addition of common carp (Cyprinus carpio) and artificial feed in rohu (Labeo rohita) ponds. We analyzed the relationships among four datasets on different components of the pond food web (water quality, food availability, natural food intake, and fish growth and production) with the aim to examine the effects of the addition of common carp and/or artificial feed on the different components of the pond food web, and to analyze the nature and strength of the interactions between these components. We used redundancy analysis (RDA) to investigate these effects and interactions. We found that the addition of common carp increased bio-available N and P in the water column. Artificial feed addition increased N and P only in the presence of common carp. N and P increases were more pronounced in the presence of 0.5 than in the presence of 1 common carp m−2. Plankton availability was strongly positively correlated with bio-available N and P. Phytoplankton availability correlated strongest with PO4–P, and zooplankton availability correlated strongest with PO4–P and DO. Natural food intake in rohu was positively correlated with plankton availability in the pond water and rohu growth was also positively correlated with natural food intake. Rohu preferred plankton over artificial feed, which acted as a fertilizer for rohu growth. Common carp preferred artificial feed over natural food and its growth was higher in the presence of artificial feed and negatively correlated with natural food availability

Functional feeding traits as predictors of invasive success of alien freshwater fish species using a food-fish model

Invasions of Ponto-Caspian fish species into north-western European river basins accelerated since the opening of the Rhine–Main–Danube Canal in 1992. Since 2002, at least five Ponto-Caspian alien fish species have arrived in The Netherlands. Four species belong to the Gobiidae family (Neogobius fluviatilis, Neogobius melanostomus, Ponticola kessleri, and Proterorhinus semilunaris) and one to the Cyprinidae family (Romanogobio belingi). These species are expected to be potentially deleterious for the populations of four native benthic fish species: Gobio gobio (Cyprinidae), Barbatula barbatula (Nemacheilidae), Cottus perifretum, and C. rhenanus (Cottidae). Invasion success may be dependent on competitive trophic interactions with native species, which are enabled and/or constrained by feeding-related morphological traits. Twenty-two functional feeding traits were measured in nine species (in total 90 specimens). These traits were quantitatively linked to the mechanical, chemical and behavioral properties of a range of aquatic resource categories, using a previously developed food-fish model (FFM). The FFM was used to predict the trophic profile (TP) of each fish: the combined capacities to feed on each of the resource types. The most extreme TPs belonged to three alien species, indicating that they were most specialized among the studied species. Of these three, only P. kessleri overlapped with the two native Cottus species, indicating potential trophic competition. N. fluviatilis and R. belingi did not show any overlap, indicating that there is low trophic competition. The two remaining alien goby species (N. melanostomus and P. semilunaris) had average TPs and could be considered generalist feeders. They overlapped with each other and with G. gobio and B. barbatula, indicating potential trophic competition. This study suggests that both generalist and specialist species can be successful invaders. Since the FFM predicts potential interactions between species, it provides a tool to support horizon scanning and rapid risk assessments of alien species.</p

Sex differentiation in seasonal distribution of the starry smooth-hound Mustelus asterias

This mark-recapture study of starry smooth-hound Mustelus asterias tagged during the summer months near the Dutch coast demonstrates a large-scale spatial sex differentiation in their circannual migration patterns and small-scale spatial sex differentiation during summer. Overwintering occurs in the North Sea, English Channel and Bay of Biscay, with significantly more males in the Northern North Sea and more females in the Bay of Biscay. During summer, sheltered sea arms off the Dutch coast were almost exclusively used by adult females. In subsequent summers post-release, both sexes were mostly confined to the Southern North Sea, suggesting philopatry

Sex differentiation in seasonal distribution of the starry smooth-hound Mustelus asterias

This mark-recapture study of starry smooth-hound Mustelus asterias tagged during the summer months near the Dutch coast demonstrates a large-scale spatial sex differentiation in their circannual migration patterns and small-scale spatial sex differentiation during summer. Overwintering occurs in the North Sea, English Channel and Bay of Biscay, with significantly more males in the Northern North Sea and more females in the Bay of Biscay. During summer, sheltered sea arms off the Dutch coast were almost exclusively used by adult females. In subsequent summers post-release, both sexes were mostly confined to the Southern North Sea, suggesting philopatry

Correction to : Historical reconstruction of sturgeon (Acipenser spp.) spatiotemporal distribution and causes for their decline in North-Western Europe

Table 1 of this article was published across four pages instead of one as it was submitted. For better clarity, Table 1 presented below in a single page