Juvenile Fish Habitat Research Articles

Hiding and feeding in floating seaweed: Floating seaweed clumps as possible refuges or feeding grounds for fishes

Floating seaweed is considered to be an important habitat for juvenile fishes due to the provision of food, shelter, a visual orientation point and passive transport. The importance of the presence of the highly dynamical seaweed clumps from the North Sea to juvenile neustonic fishes was investigated by analysing both neuston samples (without seaweed) and seaweed samples concerning fish community structure, and length-frequency distributions and feeding habits of five associated fish species. While the neustonic fish community was mainly seasonally structured, the seaweed-associated fish community was more complex: the response of the associated fish species to environmental variables was species specific and probably influenced by species interactions, resulting in a large multivariate distance between the samples dominated by Chelon labrosus and the samples dominated by Cyclopterus lumpus, Trachurus trachurus and Ciliata mustela. The results of the stomach analysis confirmed that C. lumpus is a weedpatch specialist that has a close spatial affinity with the seaweed and feeds intensively on the seaweed-associated invertebrate fauna. Similarly, C. mustela juveniles also fed on the seaweed fauna, but in a more opportunistic way. The shape of the size-frequency distribution suggested enhanced growth when associated with floating seaweed. Chelon labrosus and T. trachurus juveniles were generally large in seaweed samples, but large individuals were also encountered in the neuston. The proportion of associated invertebrate fauna in their diet was of minor importance, compared to the proportions in C. lumpus. Individuals of Syngnathus rostellatus mainly fed on planktonic invertebrates but had a discontinuous size-frequency distribution, suggesting that some of the syngnathids were carried with the seaweed upon detachment and stayed associated. Floating seaweeds can therefore be regarded as ephemeral habitats shared between several fish species (mainly juveniles) that use them for different reasons and with varying intensity.

Marine nurseries and effective juvenile habitats: concepts and applications

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 312:291-295 (2006) - doi:10.3354/meps312291 Marine nurseries and effective juvenile habitats: concepts and applications Craig P. Dahlgren1,*, G. Todd Kellison2,10, Aaron J. Adams3, Bronwyn M. Gillanders4, Matthew S. Kendall5, Craig A. Layman6, Janet A. Ley7, Ivan Nagelkerken8, Joseph E. Serafy9 1Perry Institute for Marine Science, 100 N. US Highway 1, Suite 202, Jupiter, Florida 33477, USA 2Biscayne National Park, 9700 SW 328th Street, Homestead, Florida 33033, USA 3Mote Marine Lab, Charlotte Harbor Field Station, PO Box 2197, Pineland, Florida 33945, USA 4Southern Seas Ecology Laboratories, Darling Building, School of Earth and Environmental Sciences, University of Adelaide, South Australia 5005, Australia 5NOAA Biogeography Team, 1305 East West Highway, Silver Spring, Maryland 20910, USA 6Department of Ecology and Evolutionary Biology, Yale University; PO Box 208106, New Haven, Connecticut 06520-8106, USA 7Australian Maritime College, PO Box 21, Beauty Point, Tasmania 7270, Australia 8Department of Animal Ecology and Ecophysiology, Institute for Water and Wetland Research, Radboud University Nijmegen, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands 9National Marine Fisheries Service, Southeast Fisheries Science Center, 75 Virginia Beach Drive, Miami, Florida 33149, USA 10Present address: National Marine Fischeries Service, Southeast Fisheries Science Center, 75 Virginia Beach Drive, Miami, Florida 33149, USA *Email: cdahlgren@perryinstitute.org ABSTRACT: Much recent attention has been focused on juvenile fish and invertebrate habitat use, particularly defining and identifying marine nurseries. The most significant advancement in this area has been the development of a standardized framework for assessing the relative importance of juvenile habitats and classifying the most productive as nurseries. Within this framework, a marine nursery is defined as a juvenile habitat for a particular species that contributes a greater than average number of individuals to the adult population on a per-unit-area basis, as compared to other habitats used by juveniles. While the nursery definition and framework provides a powerful approach to identifying habitats for conservation and restoration efforts, it can omit habitats that have a small per-unit-area contribution to adult populations, but may be essential for sustaining adult populations. Here we build on the nursery concept by developing a framework for evaluating juvenile habitats based on their overall contribution to adult populations, and introduce the concept of Effective Juvenile Habitat (EJH) to refer to habitats that make a greater than average overall contribution to adult populations. KEY WORDS: Habitat · Juvenile · Nursery · Nearshore Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 312. Online publication date: April 24, 2006 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2006 Inter-Research.

Availability, usage and expected contribution of potential nursery habitats for the California halibut

Coastal ecosystems have been identified as important nursery habitats for many of the world's fishery species. Beyond this, there remain many questions about what exactly constitutes high-value, even critical, habitat for juvenile fish. A first step in investigating nursery habitat value should be to catalogue the spatial coverage (availability) of all potential nursery habitats as well as the distribution (usage) of juvenile fish within those habitats. We conducted two years of fall surveys in the nearshore areas of San Diego County, CA, examining the spatial distribution of 0-group California halibut, Paralichthys californicus. The database generated by 527 otter trawls and block-net seine collections was used to produce a series of models employing regression trees to study the abiotic factors (water column and bottom features) that affect juvenile distributions. Along the exposed coast, highest 0-group densities (0.002–0.008 individuals/m 2 (indiv/m 2)) occurred where temperatures exceeded 21.5 °C (2003), and at depths between 3.3 and 5.2 m (2004). Within protected embayments, densities were higher at depths less than 1.5 m (0.054–0.430 indiv/m 2) and, in 2004, inside channeled marsh estuaries (0.156 indiv/m 2). The spatial coverage of potential nursery habitats was calculated using a Geographic Information System (GIS) database, and the total number of resident 0-group halibut within each site was estimated (habitat area × juvenile halibut density) as a proxy for expected contribution of halibut advancing to the adult stock from each nursery. Although 85% of the potential nursery habitat area occurred along the exposed coastline, 69% (2003) to 58% (2004) of 0-group halibut resided in protected embayments. Embayment contribution is much greater in the southern half of the study region, largely due to Mission and San Diego bays. We conclude that all nursery habitat types demonstrate the potential to contribute significantly to stock fitness, and that in general, shallow and better-protected habitats are highly utilized nurseries for 0-group halibut and deserve special attention in management or conservation efforts.

Juvenile sciaenid fishes respond favorably to Delaware Bay marsh restoration

Former salt hay farms in Delaware Bay have been the site of extensive restorations aimed at restoring tidal flow to the sites, encouraging Spartina alterniflora (smooth cordgrass) recolonization and creating high-quality juvenile fish habitat. We assessed the 234 ha Dennis Township restoration site as habitat for juvenile Cynoscion regalis (weakfish), Leiostomus xanthurus (spot) and Micropogonias undulatus (Atlantic croaker) by comparing abundance, prey types consumed, stomach fullness and condition factor at the restored site and at a nearby reference marsh, Moores Beach. The three sciaenid species were equally or more abundant at the restored marsh. Measures of feeding were generally equal or higher at the restored site and stomach fullness was equal to or significantly higher at the restored marsh compared with the reference marsh. Fish condition, as measured by predicted weight-at-length, was generally at least equal between the sites and was occasionally higher at the restored site. At both sites, a seasonal pattern typical of mid-Atlantic estuaries of recruitment, ontogenetic change in food habits and emigration of transient fishes was apparent. Three years following restoration, the Dennis Township site provided equivalent to enhanced conditions for feeding and growth for large numbers of juvenile sciaenid fish, compared to a nearby reference site.

Distribution and feeding of juvenile fish on invertebrates in littoral reed (Phragmites) stands

Abstract – Complex interactions between fish predators and their prey have been found in structurally complex habitats built by submerged macrophytes. In contrast, the role of comparably structured littoral reed stands in shaping biotic interactions has not been investigated. We hypothesised that reed stands may be a valuable feeding habitat for juvenile fish, and that perch and roach may segregate along the spatial and dietary niche dimensions between reed and open water habitats. In contrast, the protection effect of reed against predators was assumed to be rather low because of the lower plant volume infested in reed when compared with submerged macrophytes. We analysed biomass and growth of juvenile (age 0 and age 1) perch and roach in littoral reed habitats and in open water habitats in front of the reed in the shallow Lake Müggelsee over 4 months in 2000. Sampling was conducted by point‐abundance electrofishing over the full diel cycle (day, dusk, night, dawn). Zooplankton and benthos biomasses were determined in both habitats as well, and habitat‐specific diet of fish was assessed during day and night. Roach were more frequent than perch in both habitats. Food of roach included a higher proportion of zooplankton, whereas perch fed more on macroinvertebrates. Overall, diet overlap between the fish groups was high. Diel distribution of fish did not follow the expectations of habitat segregation between perch and roach. Instead, the function of reed as refuge habitat against littoral piscivores (mainly birds) may have caused the strong daytime preference for reed in almost all fish groups, which was partly upset by roach at night. The higher behavioural plasticity of roach may explain their good performance even under the conditions of high structural complexity.

Predation on post-settlement winter flounder Pseudopleuronectes americanus by sand shrimp Crangon septemspinosa in NW Atlantic estuaries

Assessing predation risk is necessary to evaluate the functional significance of estuaries as critical nursery habitats for juvenile fishes. The objective of this study was to quantify rates of sand shrimp Crangon septemspinosa predation on post-settlement winter flounder Pseudopleuronectes americanus in 3 NW Atlantic estuaries: Narragansett Bay (Rhode Island), Niantic River (Connecticut), and Navesink River/Sandy Hook Bay (New Jersey). Sand shrimp were collected from designated study sites in late spring and summer (2001 and 2002), and their stomach contents were analyzed with immunological assays to detect the presence of juvenile winter flounder in the diet. The results indicate that sand shrimp are possible sources of intense predator-induced mortality for newly settled winter flounder. There was a direct relationship between the incidence of winter flounder in shrimp stomachs and the body size and density of shrimp. The importance of sand shrimp as a mortality fac- tor for winter flounder in the Navesink River/Sandy Hook Bay was minimal due to the low abundance of large shrimp (≥30 mm total length). Conversely, in Narragansett Bay and the Niantic River preda- tion rates were high because of the presence of large shrimp during peak winter flounder settlement periods (May to early June), when the flatfish are small in size and vulnerable to predation. Thus, sand shrimp predation rates on winter flounder depend on the relative spatial and temporal overlap between the settlement of flounder to the benthos and the movement patterns of large shrimp that migrate into shallow estuaries to reproduce and feed.

Indo-Pacific seagrass beds and mangroves contribute to fish density and diversity on adjacent coral reefs

There is a long-standing debate whether mangrove and seagrass habitats in the Indo- Pacific region function as nurseries for coral reef fishes. We studied the use of all major shallow-water habitat types by juvenile coral reef fish using visual census surveys at 4 islands along the Tanzanian coast (East Africa) and at the island of Grande Comoros (Comoros archipelago). We investigated the value of mangroves, seagrass beds, coral reefs, macroalgae and intertidal flats as a juvenile habitat for fish by studying density distribution patterns of juveniles and adults of 76 reef fish species in these habitats. We assessed (1) which part of the reef fish community used mangrove-seagrass habitats as juvenile or adult habitats, (2) whether adult fish densities and diversity on adjacent reefs were related to the presence of these shallow habitats, and (3) whether adults of species that use these habitats when juvenile were less abundant on coral reefs situated far away from these juvenile habitats. Sea- grass beds and coral reefs were the most important juvenile fish habitats. Ontogenetic migrations between seagrass beds and reef habitats possibly occur, since several species showed their highest juvenile densities on seagrass beds, whereas adults showed their highest densities on reefs adjacent to these seagrass beds. The presence of areas with seagrass beds positively influenced adult densi- ties of many reef fish species on adjacent coral reefs. Of the 36 fish species whose juveniles were observed in seagrass beds along the Tanzanian coast, 32 species were absent from or showed low densities on coral reefs of the island of Grande Comoros (lacking seagrass beds or mangroves). On reefs far from seagrass beds and mangroves along the Tanzanian coast, 25 of these 36 species were absent or showed low densities in comparison with reefs adjacent to these habitats.

Application of the nursery-role hypothesis to an estuarine fish

In this paper, we address a recent article by Beck et al. (2001, Bioscience 51:633-641), in which it was proposed that the value of a juvenile fish habitat could be most efficiently measured as the production of adults from each juvenile habitat on a per area basis. Based upon retrospective microchemical analysis of otoliths, we calculated this metric (nursery value) for tidal freshwater and brackish littoral habitats of an estuarine fish, white perch Morone americana, in the Patuxent River estuary, Maryland (USA). In dominant year-classes of the population, brackish habitats had the high- est nursery value, whereas in all other year-classes, freshwater habitats had the highest nursery value. This result highlights the importance of inter-annual variability in habitat use, and conse- quently presents a challenging trade-off for applying the nursery-role hypothesis to facilitate habitat conservation and management. Despite this limitation, the nursery-role hypothesis provides an enriched and much needed perspective on population connectivity.

Comparison of growth and condition indices of juvenile flatfish in differentcoastal nursery grounds

Coastal and estuarine ecosystems in the eastern channel and southern bight of the North Sea provide nursery habitats for juvenile fishes. We examined the age 0-group juveniles of three flatfish speciesSolea solea, Limanda limanda and Pleuronectes platessa, collected in five nursery areas with different characteristics (three sites located near small estuaries and two affected by important inputs of industrial effluents), to evaluate habitat influence on their growth and condition. We measured a biochemical index (RNA:DNA ratio), a morphometric index (Fulton’s K condition index), plus a recent growth index (marginal otolith increment width) on each individual (about 3 months old), collected during surveys in autumn 1999. The three indices displayed few significant differences among the five nursery sites and different patterns for each species. We suggest that the investigated nursery areas provide habitats of equivalent quality for the age 0-group juvenile flatfishes in spite of different anthropogenic disturbances. On the other hand, this study focuses on the importance of using different biological indicators to assess habitat quality and environmental stressors in coastal areas.

Characterization of the marine fish assemblage associated with the nearshore hardbottom of Broward County, Florida, USA

Some shallow (<7 m, water depth) nearshore hardbottom areas of southeast Florida have been reported to function as important juvenile fish habitat. Much of this area has been impacted by one or more local beach renourishments (sand fill to offset erosion). We characterized the nearshore fish communities and compared the fish assemblages adjacent to renourished beach to those adjacent to never-renourished beach along a 30-km stretch of coastline, primarily in Broward County, using three visual census methods. Two hundred transect-counts, 100 point-counts and 98 rover-diver counts were completed during June–August 2001. In transect- and point-counts, abundance of all fish species and their sizes were recorded; the rover-diver counts consisted of a simple species list. In total, 164 species and over 72,000 fish were recorded. The highest number of species (145) was recorded with the rover-diver counts. The transects-counts had 118 species and 109 species were recorded on the point-counts. With either all the sites adjacent to renourished beach pooled and compared to the pooled never-renourished sites or individual comparisons amongst renourished and neighboring never-renourished sites, no consistent differences were noted in fish abundance or species richness (ANOVA) or among fish assemblage structure (MDS plot of Bray–Curtis dissimilarity indices). However, although the data show no obvious distinct difference between the renourished and never-renourished sites, due to several important confounding factors (e.g., census methodology, longshore movement of sand fill) and the absence of baseline data prior to any renourishment, it would be premature to translate these results into management strategies. The assemblage structure, in terms of percentage of juvenile fish (<5 cm) as well as percent contributions by family, was similar for the point-counts and transect-counts. However, in mean density per m 2 of substrate, greater abundance and greater species richness values were recorded with the transect-counts than with the point-counts. Newly settled and early juveniles were the dominant component (>84%) of the inshore fish community, consisting primarily (>90%) of grunts (Haemulidae). After the grunts, the wrasses (Labridae) at about 5%, and damselfish (Pomacentridae) at roughly 2% were the predominant families. It is clear from this study and others that the nearshore hardbottom of Broward County is an important juvenile fish habitat, especially for grunts. However, the nearshore hardbottom does not appear to be obligate habitat for these fishes as fishes associated with this area are, apparently, not unique to the nearshore hardbottom either in species or ontogenic stage.

Relative importance of interlinked mangroves and seagrass beds as feeding habitats for juvenile reef fish on a Caribbean island

Mangroves and seagrass beds are important daytime shelter habitats for juvenile Caribbean reef fish species, but little is known about their relative importance as feeding grounds. In the present study, we tested the degree to which these 2 habitats are used as a feeding ground for 4 nocturnally active fish species on Curacao, Netherlands Antilles. Stable isotope analysis was used as a technique to distinguish between feeding in mangroves and seagrass beds. Individuals of the 4 species which were sheltering during the daytime in permanently inundated fringing mangroves subject to low tidal exchange showed a stable carbon isotope signature indicative of a mixed diet composed of crustaceans from mangroves as well as seagrass beds, with the contribution from mangrove food items lying between 57 and 92%. However, individuals of the same species sheltering on adjacent (<50 m distance) seagrass beds during the daytime showed a carbon signature indicative of almost exclusive feeding on seagrass beds. This indicates that 2 different subpopulations probably exist for the 4 species studied: a population of fishes sheltering in mangroves during the daytime and feeding primarily in the mangroves and secondarily in the seagrass beds at night, and a population of fishes of the same species sheltering in adjacent seagrass beds during the daytime and feeding primarily in seagrass beds at night.

Application of habitat modelling in river rehabilitation and artificial habitat design

Over the last years there has been a growing interest in river rehabilitation and the design of artificial habitat funded by an increasing knowledge of the importance of physical habitat for populations of Atlantic salmon and trout. With the current developments in two- and three-dimensional computational hydraulics, a capability of predictive hydraulic modelling has been established and modelling and analysis of the artificial habitats are thereby possible as a part of the planning stage. This have been utilised in a project to integrate hydraulic modelling tools, habitat modelling tools, river editing tools and visualization tools into a system for analysis and design of river rehabilitation projects and artificial habitat construction. Current work includes development a river editing system where the user can edit the bathymetry of the river manually or automatically by inserting standardised habitat features like current deflectors or weirs, and a strengthening of the hydraulic and habitat models to include effects of winter temperatures and ice in the analysis. Early versions and prototypes of the system have been used in the design of artificial habitat for juvenile fish and in construction of migratory habitat for adult Atlantic salmon through weir basins.

The importance of physical and biogenic structure to juvenile fishes on the shallow inner continental shelf

Fish-habitat relationships on the shallow inner continental shelf were quantified with video sled and metered beam trawl on Fenwick and Weaver shoals offshore of Maryland and Delaware, U.S. These areas provide megascale physical relief and habitat complexity, but for juvenile fishes, mesoscale and microscale habitat is very important particularly as refuge from predation. At these smaller scales, much of the relief on the inner continental shelf is contributed by bedforms or sand waves and biogenic structures such as tubes, shell beds, or pits. A quantitative association for juvenile fishes between and within benthic habitats was found and related primarily to bedform size and amount of biogenic structure. The incidence of fishes was about four-times higher for large bedforms (> 30 cm wavelength and about 10 cm crest height) relative to smaller bedforms (<30 cm wavelength and about 5 cm crest height). For biogenic structure, going from high patch-mat tube densities to lower densities or no biogenic structure increased fish incidence by 5.4 and 3.3 times, respectively. The significant relationships of fishes with bedform size and density of biogenic structure indicated that seemingly small differences in physical structure of a habitat can make the difference between unacceptable and essential habitat for juvenile fishes. Proximity of complex and simple habitats was important in the diel use of habitat and in balancing pressure of refuge from predation provided by complex habitats with foraging for increased resources available in simpler habitats. During the day, spatially complex habitats comprised ofDiopatra andAsabellides tube mats had about twice as many fishes relative to bare sandy habitats (8.3–9.9 versus 4.0–4.1 fishes 100 m−2, respectively). At night, the pattern was reversed with more fishes present in the bare sandy habitats (12.4–13.5 versus 5.6–8.7 fishes 100 m−2). Some fish, such asAmmodytes spp., were very habitat specific and occurred only on dynamic coarser sands near the top of the shoals. Others, such asUrophycis regia, showed less habitat preference and occurred in all habitats during both day and night. Combining the effects of physical relief and biogenics, the habitat with the highest incidence of fishes had large bedforms with some biogenic structure. More emphasis needs to be placed on quantifying the relationship between fishes and their habitats for the fisheries management concept of essential fish habitat to develop into an effective tool on the inner continental shelf. The juvenile life history stages need to be emphasized because fish-habitat interactions are the strongest for these stages and may be the most ecologically important.

Diurnal Fish Density in Relation to Seagrass and Drift Algae Cover in Tampa Bay, Florida

To assess the relationship between fish density and seagrass and drift algae cover on a small geographic scale, we collected quantitative data on fish and vegetation communities during daylight hours near the mouth of the Little Manatee River, Tampa Bay, Florida. In 1991, fish were collected with two types of sampling gear, a 120-m long-haul seine and 1-m2 roving dropnets. Seagrass and drift algae cover in each sampled area was categorized as none, sparse, moderate, or dense. Despite evident gear bias, sampling with both types of gear produced similar overall fish densities. Anchoa mitchilli, Lagodon rhomboides, and Syngnathus scovelli were the most abundant or frequently collected species, regardless of gear type. Densities of 12 of the 20 most abundant species were significantly related to either seagrass or drift algae cover or both. When fish density-vegetation cover relationships were significant, the greatest fish densities always occurred in either dense or moderate covers of seagrass or drift algae. Densities of L. rhomboides and Orthopristis chrysoptera were positively related to the level of seagrass and drift algae cover in samples collected with both types of gear, but significant relationships between the densities of other species and the level of vegetation cover generally varied by vegetation or gear type (or both). Density of Eucinostomus gula peaked in moderate seagrass and declined at higher and lower levels of cover. Total fish density was similar at sites dominated by either drift algae or seagrass but was significantly reduced at sites with little cover from either vegetation type. We conclude that both seagrass and drift algae are essential habitats for juvenile and small adult fish in Tampa Bay and that fish density on a small geographic scale is strongly related to vegetation cover. Drift algae may form an important alternate habitat for fish during winter months, when levels of seagrass cover are lowest and those of drift algae are highest.

Effect of aquatic mosses on juvenile fish density and habitat use in the regulated River Suldalslågen, western Norway

AbstractTwo morphologically distinct moss communities were found in the River Suldalslågen. The liver moss community consists of species which form a dense mat on the bottom, while the river moss (Fontinalis) community forms long tufts. Moss growth has increased since hydropower regulations due to reduced floods and increased winter flows. Increased moss cover affects the bottom structure, as well as intra‐gravel and near‐bottom hydraulics. We studied densities of juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) by electrofishing and habitat selection by direct underwater observation, in areas with natural moss cover compared with areas where mosses were experimentally removed. Areas with dense mats of liver mosses held lower densities of young of year (YoY) and older salmon parr than areas where liver moss had been removed. No differences in densities of YoY salmon were found between areas with and without Fontinalis. For older salmon, parr results were inconclusive. In some samples more and in others fewer fish were found in areas with Fontinalis moss removed. For trout, densities were higher in areas with Fontinalis, while results for liver moss were inconclusive. No major differences were found with regard to microhabitat selection between areas with and without river moss, suggesting that habitat quality in these areas was similar during summer, except with respect to substrate. Salmon held more exposed positions in areas without liver moss, but this is mainly attributed to different habitat availabilities. It is concluded that the relative increase in liver mosses in the River Suldalslågen has a negative impact on juvenile Atlantic salmon fish density. Copyright © 2002 John Wiley &amp; Sons, Ltd.

An Assessment of the Feeding Success of Young-of-the-Year Winter Flounder (Pseudopleuronectes americanus) near a Municipal Pier in the Hudson River Estuary, U. S. A.

We examined feeding success of young-of-the-year winter flounder (Pseudopleuronectes americanus Walbaum) (20–50 mm TL) around a large, municipal pier in the Hudson River estuary, USA. Replicate, 3-h feeding experiments were conducted using benthic cages (0.64 m2) deployed under, at the edge, and outside of the pier during late spring and early summer in 1998 and 1999. Significantly more winter flounder caged under piers had empty stomachs ( $$\bar x$$ =71.9%) than at the edge or in open water ( $$\bar x$$ =29.2% and 14.4%, respectively). Feeding intensity was significantly higher outside of the pier ( $$\bar x$$ =0.40%) than the edge or under the pier ( $$\bar x$$ =0.19% and 0.03%, respectively). Simultaneous with feeding experiments, benthic core samples were collected adjacent to cages. Variability was high, but abundances of prey were consistently higher under the pier ( $$\bar x$$ =200.14±113.3 SD in 1998; 335±290.2 in 1999) than at the edge ( $$\bar x$$ =126.6±50.2 in 1998; 70.8±68.5 in 1999) or in open water ( $$\bar x$$ =53.4±16.1 in 1998; 123.8±193.9 in 1999). No significant differences in prey biomass were determined, suggesting that small, numerous prey were available under the pier and fewer, larger taxa were present at the edge and outside. Data indicate that feeding is suppressed among young-of-the-year winter flounder caged under piers in spite of sufficient prey available. Based on these and other experiments we submit that areas under piers are not suitable long-term habitats for juvenile fish because they interfere with normal feeding activities.

Groyne-heads as potential summer habitats for juvenile rheophilic fishes in the Lower Oder, Germany

The present study aims to test the hypothesis that artificial structures, the riverward ends of groynes or groyne-heads, function as potential habitats for young rheophilic fish. The investigation was undertaken in summer 1998 on the River Oder, in the Polish-German National Park ‘Lower Oder Valley’, eastern Germany. Fish were caught on groyne-heads using point abundance sampling by electrofishing and compared with specimens collected by the same method in seven additional mesohabitats (navigable side-canal, sandy river bank, riprap, deep groyne, sandy groyne, connected backwater, polder waterbody). Groyne-heads were inhabited by bottom-oriented 0+ rheophilic stone loach Barbatula barbatula (L.) and burbot Lota lota (L.), whereas 0+ rheophilic cyprinid fish were more abundant in shallow-sloped, sandy shoreline mesohabitats. Of all sites investigated, 1+ juveniles were most abundant on groyne-heads, in which the rheophilic group formed a large proportion of the total catch. From our results we conclude that groyne-heads constitute an important habitat for both 0+ and 1+ juvenile rheophilic fish during summer-time, but that its use is limited by the stochastic availability due to varying discharges.

Why do juvenile fish utilise mangrove habitats?

Three hypotheses to discern the strong positive association between juvenile fish and mangrove habitat were tested with field and laboratory experiments. Artificial mangrove structure in the field attracted slightly more juvenile fish than areas without structure. Artificial structure left to accumulate fouling algae attracted four-times the total number of juvenile fish than areas without structure or areas with clean structure. Community composition of fish attracted to structure with fouling algae was different when compared with areas with no structure or clean structure; five species were attracted by structure with fouling algae whilst two species were associated with structure regardless of fouling algae. Algae were linked to increased food availability and it is suggested that this is an important selection criteria for some species. Other species were apparently attracted to structure for different reasons, and provision of shelter appears to be important. Predation pressure influenced habitat choice in small juvenile fish in laboratory experiments. In the absence of predators, small juveniles of four out of five species avoided shelter but when predators were introduced all species actively sought shelter. Large fish were apparently less vulnerable to predators and did not seek shelter when predators were added to their tank. Feeding rate was increased in the mangrove habitat for small and medium-sized fish compared with seagrass beds and mudflats indicating increased food availability or foraging efficiency within this habitat. Larger fish fed more effectively on the mudflats with an increased feeding rate in this habitat compared with adjacent habitats. The most important aspect of the mangrove habitat for small juvenile fish is the complex structure that provides maximum food availability and minimises the incidence of predation. As fish grow a shift in habitat from mangroves to mudflat is a response to changes in diet, foraging efficiency and vulnerability to predators.

Ictiofauna juvenil de fondos blandos durante la transición de "secas" a lluvias en la costa pacífica de Colombia

Based on ichthyoplancton research along the southern Pacific coast of Colombia, which revealed high concentrations of fish larvae in this area, a juvenil fish survey was conducted in shallow waters « I O meter water depth) during the transition period from dry to wet season (July 21 to July 31 , 1 994). The twelve sampling stations, were distributed in fron! of the four main of this coastline, where the highest concentrations of juvenile fish were expected. At each sample station, three 15 min oblique hauls were conducted every g hours using a bottom trawl for juvenile fish of 4 mm mesh size; 53 species in 20 families were identified (21 not known as adults in the area). They were distributed as follows: Sciaenids (11), Ariids (7), Pristigasterids (6), Engraulids (6), Carangids (5), Clupeids (2), Soleids (2), Batrachoidids (2), Tetraodontids (2), Urolophids (1), Polynemids (1), Synodontids (1) and Taeniopedids (1). This ichthyofauna is cornmon in open tropical estuaries. Abundances and biomass were 0.35 ind.lm3 and 1 .61 g/m3 respectively, higher than in other tropical shallow water systems. The niches of ihe 25 species whose abundance (A) and biomass (B) contributed 97.5 % and 93.3 % respectiveIy to the total catches were: (1) pelagic-neritic (53 % A, 34 % B); (2) demersal-pelagic (1 8 % A, 28 % B); (3) demersal (22 % A, 23 % B) and (4) benthic (8 % A, 1 5 % B). The Shannon-Weaver diversity index in the four subareas ranged from 0.89 to 1.1, within the range reported for other tropical American estuarine systems. An ordination analysis (non-metric Multi-Dimensional Scaling, MDS) of the data revealed significant differences in the species composition between day and night and an importan! association among sorne species. Station position, tides and environmental parameters did not markedly influence species composition. It is concluded Ihat: (1) The area surveyed represents an important nursery and growth habitat for juvenile fish of many species that are more common as adults over soft bottoms in deep waters of the continental shelf; (2) this shallow soft bot- 10m fish cornmunity is c1early separated from those species inhabiting intertidal salt marshes, interior bays and mangrove areas; and (3) sorne of the fish species commonly living in rocky and reef habitals in adult stages, spend their juvenile stage over shallow water soft bottoms. These results confirm a rather strong niche separation between juvenile and adults fish species stages in tropical waters.

Tidal effects on habitat selection and aggregation by juvenile pollock Pollachius virens in the rocky intertidal zone

Fucoid macroalgae in the rocky intertidal zone are a potentially important foraging and refuging habitat for juvenile fishes. A dominant feature of this habitat is that its availability changes with the tides. Vegetated habitat availability changes in many other systems as well, yet little is known about the effects of these changes on animal distributions. We addressed this problem by studying young-of-the-year pollock Pollachius virens using visual transect surveys in the rocky intertidal zone. We examined tidal effects on pollock distribution, their depth and habitat preferences and the relationship between habitat use and patterns of aggregation. Nearly all the pollock population in our study area moved into the intertidal zone and alternated between aggregation in the open and dispersal in the algae. On rising tides, pollock moved from the subtidal zone to the open intertidal zone in large schools then dispersed among available depths and throughout algal habitats in small schools or as solitary flsh When in algae, pollock preferred the dense algal habitat over the sparse algal habitat. On falling t~des , pollock schooled In the open habitat In downshore intertidal and subtidal zones. The main seasonal difference, from early to late summer, was an overall distributional shift towards the downshore open habitat and thv subtidal zone and a preference for greater depths in the intertidal zone. These results support the hypothesis that pollock were using both refuging and schooling antipredator tactics during intertidal zone migrations. We suggest that rocky shores are important nurseries for juvenile pollock. K E Y WORDS: Habitat selection . Habitat availabil~ty . Refuging . Schooling . Marine fish nursery . Rocky intertidal zone . Pollachius wrens. Pollock . Macroalgae . Gadidae . Visual transects