Juvenile Pelagic Fish Research Articles

Empirical verification of feeding selectivity of larval and juvenile pelagic fishes using in-situ zooplankton assemblages

Most studies on the feeding ecology of larvae and juveniles of commercially important pelagic fishes have used field-based approaches. However, due to possible biases related to net sampling, it is uncertain whether the results obtained from those studies truly represent the situation of live fish in the sea. Here we investigated the feeding ecology of pelagic fishes through a laboratory experiment minimizing the biases inherent in field net sampling. In the experiment, hatchery-reared juvenile chub mackerel (Scomber japonicus) and larval/juvenile Japanese anchovy (Engraulis japonicus) were fed with wild-caught zooplankton assemblages collected from around Hakatajima Island in the Seto Inland Sea, Japan. The relationships between fish size and prey number in the gut, and the selectivity on each prey organism were determined. As a result, in both species, prey number and size increased with body size, and the fish showed strong selectivity for crustaceans including copepodites and adults of copepods. Our data has also clearly indicated that both species can selectively prey on preferred foods that are rare while avoiding non-preferred foods that are abundant. These results, which substantially accord with reports from previous field studies, will not only help field scientists make a convincing interpretation of their data, but also open the possibility of further laboratory studies on detailed mechanisms of the feeding selectivity of larval/juvenile pelagic fishes.

Stable isotopes reveal sargassum rafts provide a trophic subsidy to juvenile pelagic fishes

Floating macroalgal rafts can form extensive habitat in coastal waters and support abundant and diverse communities of juvenile fishes. Rafts comprised predominantly of Sargassum spp. are common within the Ningaloo Coast World Heritage Area, Western Australia, and may be an important transitional habitat for pre-settlement coastal fishes. We had three aims: 1) to quantify the diversity and abundance of fishes associated with Sargassum rafts; 2) to use stable isotopes to determine whether fishes associating with rafts preyed on nearby plankton or on prey associated with Sargassum; and 3) to determine whether the food web fishes relied upon was ultimately supported by primary production of Sargassum or planktonic phytoplankton (measured as seston). Fishes and macroinvertebrates associating with Sargassum rafts, zooplankton, and seston were collected at six sites in the Ningaloo Marine Park and analysed for δ15N and δ13C, and the diets of the most common fishes were analysed using MixSIAR. Fishes were more abundant around macroalgal rafts than in open water. Eleven species of juvenile fishes associated with Sargassum rafts, with one species (Histrio histrio (Antennariidae)) represented by juveniles and adults. Four of the five fishes most sampled around Sargassum spp. rafts, (Abudefduf vaigiensis (Pomacentridae), Helotes sexlineatus (Terapontidae), Colurodontis paxmani (Monacanthidae) and Petroscirtes breviceps (Blenniidae)) had generalist diets whose food webs were ultimately supported by greater contributions of primary production from Sargassum spp. (55–72%) than planktonic seston. H. histrio was predominantly piscivorous but still derived most (64%) of its organic matter indirectly from Sargassum spp. Our results reveal the importance of Sargassum rafts as habitat that provides shelter and food for juvenile fishes and adult H. histrio. Moreover, the prevalence of juvenile fishes associating with Sargassum rafts highlights a bidirectional form of benthic-pelagic coupling whereby macroalgae that detach from the benthos provide habitat and a trophic subsidy for pelagic juvenile fishes before they the transition into benthic coastal habitats.

Development of a new multiple sampling trawl with autonomous opening/closing net control system for sampling juvenile pelagic fish

A new multiple layer sampling trawl with an autonomous net opening/closing control system was developed to sample pelagic juvenile fish quantitatively. The new trawl system, based on the Matsuda–Oozeki–Hu Trawl (MOHT), has a rigid-frame 3.3m high and 2.35m wide and five nets of 11.0m length with a rectangular mouth of 2.22m×1.81m (4m2 mouth area; large-scale prototype). A cambered V-shape depressor is hung below the frame and two bridles are attached at the midpoint of the side frames. A net-release controller is used, which not only controls the net release mechanism but also records the net depth, temperature and flow rate during net towing. The controller sends stored command signals to the net release mechanism as depth settings and/or time settings and does not require any commands from the surface through a conducting cable or by acoustic signals. Two other models were constructed before the construction of the large-scale prototype, which are a small-scale prototype (2m2 mouth area) for testing the net release mechanism and a 1/4-scale model of the large-scale prototype for flume tank tests. Flume tank tests with the 1/4-scale model showed that the frame leaned forward at a tilt angle from 5 to 15 degrees at towing speeds from 0.8 to 1.4ms−1. Opened nets closed smoothly and sequentially nets were completely opened when the trigger was released by the command. Net depth rarely changed even during changes in towing speed. Sea trials both by the small-scale and the large-scale prototype demonstrated the same towing characteristics expected from the flume tank tests. The newly developed multiple layer opening/closing MOHT (MOC-MOHT) is considered to be suitable for quantitative layer sampling of juvenile fish.

Feeding ecology of pelagic fish larvae and juveniles in slope waters of the Gulf of Mexico

Stable isotope ratios of carbon (delta13C) and nitrogen (delta15N) were used to investigate feeding patterns of larval and early juvenile pelagic fishes in slope waters of the Gulf of Mexico. Contribution of organic matter supplied to fishes and trophic position within this pelagic food web was estimated in 2007 and 2008 by comparing dietary signatures of the two main producers in this ecosystem: phytoplankton [based on particulate organic matter (POM)] and Sargassum spp. Stable isotope ratios of POM and pelagic Sargassum spp. were significantly different from one another with delta13C values of POM depleted by 3-6 per thousand and delta15N values enriched by 2 relative to Sargassum spp. Stable isotope ratios were significantly different among the five pelagic fishes examined: blue marlin Makaira nigricans, dolphinfish Coryphaena hippurus, pompano dolphinfish Coryphaena equiselis, sailfish Istiophorus platypterus and swordfish Xiphias gladius. Mean delta13C values ranged almost 2 among fishes and were most depleted in I. platypterus. In addition, mean delta15N values ranged 4-5 with highest mean values found for both C. hippurus and C. equiselis and the lowest mean value for M. nigricans during both years. Increasing delta13C or delta15N with standard length suggested that shifts in trophic position and diet occurred during early life for several species examined. Results of a two-source mixing model suggest approximately an equal contribution of organic matter by both sources (POM=55%; pelagic Sargassum spp.=45%) to the early life stages of pelagic fishes examined. Contribution of organic matter, however, varied among species, and sensitivity analyses indicated that organic source estimates changed from 2 to 13% for a delta(13)C fractionation change of +/-0.25 per thousand or a delta15N fractionation change of +/-1.0 per thousand relative to original fractionation values.

Structure and biological characteristics of purse seine landings off the Lebanese coast (eastern Mediterranean)

Small pelagic fishes constitute most of the fish caught in the Mediterranean Sea and two thirds of the Lebanese landings. In this study, we present some biological parameters for the most common species collected during the fishing season. In Lebanon, juvenile pelagic fishes are attracted to floating lights and captured using purse seine nets. At least 32 species were collected but only 11 (mainly Engraulis encrasicolus, Sardina pilchardus, Sardinella aurita and Scomber japonicus) represented most of the catches. Weight–length relationships and different condition indices for common species were calculated. Most landed fishes were juveniles with a mean size of 11.3 ± 8.8 cm TL. Fish conditions exhibited a similar temporal pattern with a minimum recorded in July, probably due to interspecific food competition in the nurseries. The monitored fishing vessel landed about 15 tonnes of juvenile fishes during the 4-month fishing season while fishing effort remained constant. Size-ranges and length–frequency distributions showed that the dominant sizes ranged between 6 and 8 cm TL and that harvested fishes are smaller than the minimum size fished in neighbouring Mediterranean countries. The Lebanese purse seine fishery targets 0 age-class juvenile of many fishes in the nurseries, which is against sustainable fishing practices and has a potential impact on pelagic fish communities in the eastern Mediterranean.

Diel shifts in community composition and feeding of juvenile fishes in the pelagic area of a large shallow lake

Diel cycles of changing ambient illumination have been shown to have strong influence on fish community composition in freshwater systems, mainly due to diel habitat shifts of fish between sheltered and more exposed, but profitable sites. Low information is available, however, from diel patterns of fish community composition and feeding in the central areas of large shallow lakes where lake depth and diameter may hamper diel migrations. We studied whether there are diel differences in species abundance and shifts in feeding modes of the juvenile pelagic fish species over an 8 months sampling period in a shallow lake. The strong decline of perch numbers towards the night suggests low nocturnal activity for this species. In contrast, the nearly constant numbers of roach over the full diel cycle points to pronounced activity under changing light intensities. Increased ruffe numbers at night reflect the good adaptation of this species to feed efficiently at low light intensities. Niche segregation of fish species along the diet axis was low. There was low evidence that diel shifts in the fish community composition were attributed to diel horizontal migrations of species into or out of the pelagic zone. Because availability of preferred zooplanktonic prey was high, differential diel activity patterns of species reflect their genetically fixed, albeit varying adaptations to feed under low-light conditions, instead of being an active avoidance to reduce competition.

Newly designed quantitative frame trawl for sampling larval and juvenile pelagic fish

A newly designed frame trawl with a 5 m2 mouth area was constructed for quantitative sampling of larval and juvenile pelagic fishes and its performance was tested in the ocean. Towing depth of the trawl was easily controlled by adjusting the warp length, and the stability at depth increased with decreasing weight of the net in water. Larval and juvenile fishes were sampled using the trawl at stations in the Kuroshio-Oyashio transition region from 29 May to 24 June 1998. A total of 44 oblique tows were conducted and towing depths of the frame trawl had a high degree of stability despite the various sea conditions and the fluctuations in the ship speed. Larvae and juveniles of Japanese anchovy Engraulis japonicus and Myctophidae made up the majority of the specimens. It was concluded that the newly constructed trawl was appropriate for the quantitative sampling of pelagic fish larvae and juveniles.

Drifting objects as habitat for pelagic juvenile fish off New South Wales, Australia

The importance of drifting objects to small juvenile pelagic fish was investigated off the coast of New South Wales, Australia. Distance-related and temporal patterns in the distribution of clumps of drifting algae were investigated with 5000 m2 transects at five distances from shore (0.1, 0.5, 1, 5 and 10 km), two to three times per season for 2 years. Juvenile fish associated with drift algae were collected. Clumps of algae, predominantly Sargassum spp., were most abundant in spring, which coincided with the highest abundance of alga-associated post-flexion juvenile fish. Drift algae were also most abundant close to shore, probably due to the proximity to source and the dominant onshore winds. Fish were quickly attracted to drifting artificial objects (fish aggregation device; FADs), although the magnitude of attraction varied greatly among days. The relative abundance of small fish in open waters available to colonise FADs and differing weather conditions may explain much of this variability. More fish colonised FADs with an odour source than unscented control FADs, indicating small fish may use chemical cues to locate drifting structures. We conclude that juvenile fish actively seek drifting objects as pre-settlement habitat, which may reduce predation and enhance settlement opportunities.

Spatial overlap and distribution of anchovies ( Anchoa spp.) and copepods in a shallow stratified estuary

Juvenile pelagic fishes are integral members of many coastal river communities. Many of these systems are strongly influenced by variable wind stress and freshwater inputs that can increase heterogeneity in estuarine habitat for fishes. We use mobile sonar surveys within the Neuse River Estuary System, NC, USA to assess the distribution and behavioral patterns of juvenile anchovies, Anchoa spp. 25-65 mm TL, over a broad range of spatial scales in relation to diel and seasonal changes in water quality including stratification, hypoxic events and copepod distribution. Results from our study indicate that episodic stratification-induced hypoxic events can reduce suitable habitat volume for anchovies by more than 50%. Furthermore, our sampling suggests that hypoxia causes spatial separation between plankton and the grazing fishes. Under stratified oxygen conditions, we observe higher densities of copepods in hypoxic bottom water. Finally, we report that reductions in available habitat caused an increase in local densities of fishes and may result in increased competition for resources. These spatially explicit data are critical for developing trophic dynamic models that predict the response of fish communities to natural and anthropogenic impacts on the system.

Climate change causes contrasting trends in reproductive performance of planktivorous and piscivorous alcids

Summary 1. Eight years of observations on seabird reproductive success and oceanographic change in Tauyskaya Bay (Okhotsk Sea, north‐western Pacific) were used to evaluate the hypothesis that interannual climate change causes opposite trends in reproductive performances of planktivorous auklets (Aethia cristatella and Cyclorhinchus psittacula) vs. piscivorous puffins (Lunda cirrhata and Fratercula corniculata). 2. The climate change was assessed by examining changes in sea‐surface temperature (SST), time of permanent ice disappearance (ID), wind (WV) and current vectors (CV). Changes in the distribution of zooplankton biomass in the study region were used to assess changes in prey communities. Bird reproductive success was determined as the number of chicks fledged per nest occupied. 3. There were two distinct sets of oceanographic conditions in the study region, as reflected in the SST, ID, WV and CV. Strong northerly winds in the spring produced a late ice disappearance in the study region, whereas easterly winds determined an early ice disappearance. The patterns in ice disappearance were significantly correlated with SST anomalies during the summer. A negative SST anomaly (– 1·2 °C) defined a ‘cold’ regime, whereas a positive SST anomaly ( + 1·2 °C) defined a ‘warm’ regime. 4. Reproductive success of planktivorous auklets was negatively correlated with the SST in the western part of Tauyskaya Bay, whereas reproductive success of piscivorous puffins was positively correlated with the SST. The ‘cold’ season in 1988 was characterized by a strong in‐flow of water masses into the bay area. The ‘warm’ season in 1989 was characterized by well‐mixed warm water inside the bay that were separated from colder water masses outside the bay. Macro‐zooplankton, which were the main prey of planktivorous auklets, were more abundant during the ‘cold’ regime of the ecosystem. Meso‐zooplankton, a potential prey of juvenile pelagic fish, were more abundant during the ‘warm’ regime of the ecosystem. 5. Interannual oceanographic change probably impacts alcid reproductive performances by affecting food accessibility to planktivorous auklets and piscivorous puffins in opposite ways.

Understanding Recruitment of Lake Michigan Fishes: The Importance of Size-Based Interactions Between Fish and Zoopiankton

Mechanisms controlling recruitment of fishes appear to be strongly size dependent. It is now established that size-selective predators can dramatically reduce zooplankton size, but little is known about the effects of zooplankton size on growth and recruitment of fish through the post-larval stage. As fish grow, their optimal prey size increases; if large zooplankton are uncommon, growth rates may be reduced, prolonging vulnerability to predation or other size-dependent mortality sources and thus reducing recruitment. Most Lake Michigan fishes, including offshore species such as bloater (Coregonus hoyi) and nearshore species such as yellow perch (Perca flavescens), shift from feeding on zooplankton in their early years to feeding on benthic prey or to piscivory. Predation mortality on many larval and juvenile fishes including bloater and yellow perch has been shown to be size or growth rate dependent. As alewife (Alosa pseudoharengus) increased in abundance in Lake Michigan in the 1960s, large zooplankton declined and both bloater and perch recruitment was poor. In the late 1970s and early 1980s, alewife declined, large zooplankton increased, and strong year classes of bloater and perch were formed. Based on these dynamics and recent research on resource use, foraging behavior, and recruitment dynamics of larval and juvenile fishes, we suggest two hypotheses. First, young-of-year and juvenile pelagic fishes may have the major size-structuring effects on epilimnial zooplankton in Lake Michigan. And second, if large zooplankton are uncommon, as they were in Lake Michigan in the 1960s, growth rates and recruitment of native fishes will be reduced. Size-based interactions between fish and zooplankton appear to have important implications for growth and recruitment success of fishes.

Design and Relative Efficiency of a Bow-Mounted Pushnet for Sampling Juvenile Pelagic Fishes

Abstract A bow-mounted pushnet designed to sample near-surface young-of-the-year pelagic fishes had a significantly higher catch rate than a stern trawl previously used. The ratio of catches (numbers of fish) for simultaneous sampling with the pushnet and two identical trawls was 23:3:1. Both gear sampled the same population with respect to fish length. The pushnet requires a crew of only two, can be operated in shoal water, and multiple net installation is possible. The outboard vessel-pushnet unit is highly mobile because the frame is readily raised for cruising and swung aft, but not detached, for trailering.