Abundance Of Small Fish Research Articles

Feeding ecology of Benthosema glaciale across the North Atlantic

The glacier lanternfish Benthosema glaciale is a key myctophid with a wide distribution in the northern Atlantic. It is a species that has a strong vertical migration capacity and have the potential to move between the surface waters and mesopelagic depths in a diel cycle (DVM), mainly depending on ambient light conditions. We investigated the feeding ecology of B. glaciale across the Norwegian, Iceland, Irminger and Labrador Sea basins. An important component of Benthosema diet was various types of calanoid copepods, but with some additional variability across the deep basins. ‘House’s’ of Appendicularia were only found in stomachs from the Labrador basin and were positively selected for here. The large calanoid Calanus hyperboreus was strongly selected for in the Iceland Sea, while its smaller counterpart C. finmarchicus was negatively selected for here. Fish from the Irminger, Labrador and Norwegian Seas displayed a high number of empty stomachs while no fish stomachs were found empty in the Iceland Sea. Contrary to expectation due to being located at the highest latitude, Benthosema in the Iceland Sea had significantly higher condition factor (for both small and large fish) and liver indices compared to fish from other basins, but the abundance of small fish in the sampled population here was very low. This contrasts with the “light environment exclusion” (LEE) hypothesis, which propose that the extreme light environment at higher latitudes restricts feeding opportunities for mesopelagic fish at high latitudes. It is suggested that improved classification of prey through stomach analyses should aim to allow bioluminescent organisms to be separated from non-bioluminescent prey if feasible. This would allow improved resolution to analyse stomach contents and certainly progress our understanding of the success of myctophids across variable habitats.

Hydroacoustic survey on fish spatial distribution in the early impoundment stage of Yuwanghe Reservoir in southwest China

Understanding the fish community structure and spatial distribution characteristics is essential for appropriate reservoir fishery management, especially in the early impoundment stage of the reservoir, which could help in obtaining basic data and performing artificial adjustments to biological communities. On the basis of this concept, we conducted a survey of the fish community and distribution at the newly constructed reservoir in southwest China by using a combination of methods, including hydroacoustic survey and fish sampling. Fish sampling showed a single fish community structure (six species belonging to four families) assembled in the reservoir, and the dominant species was Pseudorasbora parva which accounted for 98.79% of the fish population. In the hydroacoustic survey, the average fish density was 318.7 ± 256.1 individuals/1,000 m2. Irregular distribution of the fish was observed in the horizontal direction. The fish densities in the head area, middle area, and tail area of the reservoir were 168.5 ± 60.1, 306.8 ± 124.7, and 696.4 ± 288.9 individuals/1,000 m2, respectively, which showed a trend of increase in fish density with an increase in distance from the dam. More than 97.3% of the fish in the vertical direction were distributed in the water layer of depth 0 to 12 m. The average total length of fish was 75.4 mm, and the mean fish biomass in the reservoir was approximately 984.8 g/1,000 m2. According to Pearson correlation analysis, the main factors affecting the spatial distribution of fish were water depth, water temperature, dissolved oxygen, total nitrogen, and plankton density. Our results suggested that the fish community structure should be adjusted by releasing native carnivorous fish to control the abundance of small fish. Moreover, filter-feeding fish such as silver carp and bighead carp should be released in the reservoir to control the plankton community; this will enable the maintenance of the reservoir ecosystem in a healthy state while increasing the economic benefits to the local area.

Movement Shapes the Structure of Fish Communities Along a Cross-Shore Section in the California Current

Pelagic fish communities are shaped by bottom-up and top-down processes, transport by currents, and active swimming. However, the interaction of these processes remains poorly understood. Here, we use a regional implementation of the APex ECOSystem Model (APECOSM), a mechanistic model of the pelagic food web, to investigate these processes in the California Current, a highly productive upwelling system characterized by vigorous mesoscale circulation. The model is coupled with an eddy-resolving representation of ocean currents and lower trophic levels, and is tuned to reproduce observed fish biomass from fisheries independent trawls. Several emergent properties of the model compare realistically with observations. First, the epipelagic community accounts for one order of magnitude less biomass than the vertically migratory community, and is composed by smaller species. Second, the abundance of small fish decreases from the coast to the open ocean, while the abundance of large fish remains relatively uniform. This in turn leads to flattening of biomass size-spectra away from the coast for both communities. Third, the model reproduces a cross-shore succession of small to large sizes moving offshore, consistent with observations of species occurrence. These cross-shore variations emerge in the model from a combination of: (1) passive offshore advection by the mean current, (2) active swimming toward coastal productive regions to counterbalance this transport, and (3) mesoscale heterogeneity that reduces the ability of organisms to return to coastal waters. Our results highlight the importance of passive and active movement in structuring the pelagic food web, and suggest that a representation of these processes can help to improve the realism in simulations with marine ecosystem models.

Fish size spectra are affected by nutrient concentration and relative abundance of non-native species across streams of the NE Iberian Peninsula

Aquatic ecosystems are strongly body-size structured with a decline of numerical abundance with increasing body size (hereafter, the size spectrum). Marine and lake fish studies have reported consistent variations of size spectra in relation to environmental conditions and biotic composition, but little is known about stream fishes. Accordingly, in this study we test several hypotheses about the effects of local water conditions, biotic introductions and cumulative pressures (measured as the IMPRESS index) on the fish size-spectrum slope (that is, the linear rate of decline of fish abundance as body size increase in a log-log scale) and the size-spectrum intercept (commonly used as proxy for carrying capacity) among 118 local fish assemblages in streams of the NE Iberian Peninsula. To our knowledge, this is the first time that an extensive river fish dataset is used in a dendritic network to cover systematic changes of size-spectrum parameters. We find that the slope and intercept of the fish size spectrum are negatively correlated with nutrient concentration (mainly total phosphorus), with a greater relative abundance of small fishes but a decline of overall carrying capacity. Moreover, fish assemblages with greater relative abundance of non-native species have flatter size-spectrum slopes. In contrast, the IMPRESS index and climate-related variables are poor predictors of the shape of the fish size spectra. This study contributes to better understanding of the main factors structuring fish assemblages in lotic environments of the Iberian Peninsula. We encourage more research on this line to further explore the use of fish size structure to evaluate the ecological health of riverine ecosystems.

Phytoplankton species distribution around the fishing ground of Oma waters, Central Maluku, Indonesia

Oma water is directly related to the Banda Sea, which causes this water to have the highest diversity of fisheries resources. Oma waters is a potential fishing ground for small and big pelagic fishes. One of the most important pieces of information on the abundance of small fishes in this area is the abundance of phytoplankton. Until now, there was not any information available on phytoplankton species composition as well as its abundance. The objective of this research was to study the species composition of phytoplankton, its abundance and, some ecological indices. Sampling was carried out in April 2017 at a 14-fixed sampling station using Kitahara plankton net. Sampling was conducted vertically from 20 meters depth to the surface water. Hydrological parameters such as temperature, salinity, pH, and water transparency were measured in-situ. Nitrate and phosphate were analyzed in a chemical laboratory in LIPI-Ambon. The total of 65 phytoplankton species was identified from the study area and can be grouped into Bacillariophycea, Dinophyceae, and Cyanophyceae. Bacillariophycea has dominated this phytoplankton species composition (70.70%). A total abundance of phytoplankton in this study area varied from 1.09 × 105 cells l -1 to 5.70×105 cells l -1. The five dominant species found in this study were Guinardia flacida, Thalassionema nitzschioides. Skeletonema costatum, Thalassiosira gravida, Chaetoceros curvisetus, and The Shannon diversity index (H’) varied from 1.447 to 2.774. The evenness index (J’) was between 0.404 and 0.738. The dominance index was from 0.262 to 0.596. Phytoplankton abundance had a positive correlation with water transparency and salinity, whereas the number of phytoplankton had a positive correlation with seawater temperature and nitrate concentration

Species composition and abundance of small fishes in seagrass beds of the Karang Congkak Island, Kepulauan Seribu National Park, Indonesia

It is extensively recognized that seagrass meadows have been defined as nursery ground for fish. In this study, we investigated species composition and abundance of small fishes in seagrass beds of Karang Congkak Island, Kepulauan Seribu National Park from November 2018 to March 2019. In total, about 10,000 individuals of 46 fish species belonging to 26 families were captured using a seine net at four fix stations. The major families graded by species number were Labridae, Apogonidae, Gobiidae, Siganidae, and Atherinidae. More than 90% of fish was juvenile and mostly economically important species and reef-associated fish. Majority of fish juveniles inhabit seagrass beds were categorized as temporary resident and regular visitors. It was observed that the top five ranked fish species in abundance were Spratelloides gracilis (33.4%), Stenatherina panatela (19.5%), Siganus canaliculatus (13.2%), Gerres oyena (11.8%) and Siganus spinus (5.9%). There was a propensity that species richness and diversity were higher in areas with higher seagrass coverage. However, two-way ANOSIM revealed fish abundance was not significantly different spatially and temporally (p>0.05). Predominant trophic function of fish were zooplanktivores and crustacivores. The present study, therefore, identified seagrass beds of Karang Congkak Island as feeding habitats and shelter for fish juveniles.

Reconstructing the build-up of a pelagic stickleback (Gasterosteus aculeatus) population using hydroacoustics

A lake wide fishing survey of Upper Lake Constance revealed that in September 2014 three-spined sticklebacks (Gasterosteus aculeatus) made up 96% of the pelagic fish community in terms of number. To narrow down the point in time when sticklebacks started to colonize the pelagic zone and to reconstruct the build-up of this unprecedented population size, we analysed 18 hydroacoustic surveys from March 2009 to March 2018. As current analysis techniques do not allow discriminating echoes from different fish species of similar size, we considered as sticklebacks all fish smaller than a certain single-target threshold minus the average abundance of small fish well before the sticklebacks invaded the pelagic zone. Our data suggest that sticklebacks colonized the pelagic zone in 2012, one year before they appeared as by-catch in test fisheries. Population density fluctuated between 1280 and 7990 individuals/ha from November 2012 to March 2018 but never fell below 2900 individuals/ha in recent years. These estimates do not include shallow waters where densities might be even higher because the nearshore zone was excluded from our surveys.

Fish and seagrass communities vary across a marine reserve boundary, but seasonal variation in small fish abundance overshadows top-down effects of large consumer exclosures

A growing number of examples indicate that large predators can alter seagrass ecosystem structure and processes via top-down trophic interactions. However, the nature and strength of those interactions varies with biogeographic context, emphasizing the need for region-specific investigations. We investigated spatial and temporal variation in predatory fish and seagrass communities across a Marine Protected Area (MPA) boundary in the Banana River Lagoon, Florida (USA), assessing trophic roles of intermediate consumers, and performing a large-consumer exclusion experiment in the MPA. Large, predatory fishes were most abundant within the MPA, while some mid-sized fishes were more abundant outside it. Small, seagrass-resident fishes, epifaunal invertebrates, and macrophytes also differed across the MPA boundary, but varied more among individual sites and seasonally. We cannot conclusively attribute these patterns to MPA status because we lack data from prior to MPA establishment and lack study replication at the level of MPA. Nevertheless, other patterns among our data are consistent with hypothesized mechanisms of top-down control. E.g., inverse seasonal patterns in the abundance of organisms at adjacent trophic levels, coupled with stable C and N isotope and gut contents data, suggest top-down control of crustacean grazers by seasonal recruitment of small fishes. Large-consumer exclosures in the MPA increased the abundance of mid-sized predatory and omnivorous fishes, but had few impacts on lower trophic levels. Results suggest that large-scale variation in large, predatory fish abundance in this system does not strongly affect seagrass-resident fish, invertebrate, and algal communities, which appear to be driven more by habitat structure and seasonal variation in small fish abundance.

Salinity shapes zooplankton communities and functional diversity and has complex effects on size structure in lakes

Changes in zooplankton community structure and function were analyzed in 24 lakes covering a wide salinity gradient (from 0.5 to 115 g l−1) in a semiarid region in northwest China. We hypothesized that species richness (S), species diversity (H), functional diversity (FD), biomass, and size of zooplankton would decrease with increasing salinity. We found that S, H, and FD did decrease with increasing salinity, whereas zooplankton sizes, size range, and biomasses did not. In fact, the sizes of microcrustaceans were mainly regulated by the abundance of small fish. Besides the impoverishment of FD, the zooplankton functional groups also varied along the salinity gradient. A shift occurred from selective raptorial to more generalist microphagous rotifers, from selective to more generalist filter feeder cladocerans, and from dominance of microphagous herbivorous copepods to microphagous carnivores. Our study indicates that the ongoing salinization of lakes with climate warming will result in important changes in the zooplankton, affecting not only the structure but also the functioning of this community. A weakened top-down control by zooplankton on phytoplankton at moderate high salinities may be an indirect consequence, leading to a worsening of eutrophication symptoms. Loss of fish at high salinities may, however, counteract this effect.

The impacts of short-term temporal factors on the magnitude and direction of marine protected area effects detected in reef fish monitoring

Marine protected areas (MPA) and in particular no-take marine reserves have been identified as important tools for the conservation of reef fish and habitats. A significant challenge of reef fish monitoring is to determine the influence of temporal factors on fish counts. Fish assemblages are dynamic and changes in activity patterns throughout the day can influence the results of surveys. While many monitoring programs account for the impacts of spatial heterogeneity on fish assemblages the effects of short-term temporal variation are less well known. In the present study, we analysed data from 197 video drops inside and outside New Caledonian MPAs and examined temporal variations in various metrics commonly used to monitor marine reserves. In addition to describing short-term temporal patterns related to time of day, tide height and state, and lunar cycle; we also examined the influence of these temporal factors on the size and direction of any MPA effects detected. Fewer Serranidae were observed at full moon than the rest of the lunar cycle and there were negative correlations between Chaetodontidae and Acanthuridae abundance and tide height. We did not find any consistent effects of time of day. Generally, variation in short term temporal factors did not affect the direction MPA effects detected but did affect the size of the effects for some metrics. For both small fish abundance and species richness, bigger differences between protected and unprotected sites were detected at high tide than low tide. These results highlight how survey results can vary with timing of sampling and have implications for developing optimal monitoring programs.

Do invasive rusty crayfish (Orconectes rusticus) decouple littoral and pelagic energy flows in lake food webs?

Linkages between habitats can strongly affect ecosystem function through exchange of energy and materials. In lake food webs, large piscivorous fishes integrate littoral and pelagic energy sources through the consumption of smaller fishes and macroinvertebrates. We tested the hypothesis that the introduction of the invasive rusty crayfish (Orconectes rusticus) decouples littoral and pelagic food webs by reducing the abundance of small fish and noncrayfish benthic macroinvertebrates, thereby shifting diets of large fish to increased dependence on littoral energy sources (i.e., crayfish). We used stable isotope ratios of C (δ13C) and N (δ15N) to examine differences in resource use between lakes with high abundance of O. rusticus (invaded) and lakes with native crayfish only or with low abundance of nonnative crayfish (uninvaded). The trophic position of juvenile crayfish was lower in invaded than in uninvaded lakes, results suggesting decreased availability of, and reliance on, macroinvertebrate prey and greater dependence on detritus and algae. In invaded lakes, diets of piscivorous fishes relied more strongly on littoral energy sources, and Walleye (Sander vitreus), an obligate piscivore, had a lower trophic position than in uninvaded lakes, results suggesting reduced availability of fish prey. In agreement with another recent stable-isotope study and observations from many smaller-scale but more mechanistic mesocosm and enclosure experiments, these results support the hypothesis that invasion by O. rusticus causes a reduction in the abundance of benthic macroinvertebrates and small fishes and consequently reduces trophic position and increases littoral dependence of large fishes.

Biotic and abiotic effects on cohort size distributions in fish

Intraspecific variation in body size is common in animals and plants. Body size affects trophic interactions like foraging ability and vulnerability to predation, which in turn affect individual fitness as well as population stability and extinction risk. Experimental and theoretical work has shown that the size distribution of individuals within cohorts is strongly influenced by intraspecific competition for resources, often leading to skewed frequency distributions. However, little is known about the effects of environmental factors such as climate and eutrophication on the cohort size‐structure of natural populations. We use a long‐term time series of scientific monitoring of a freshwater fish (European perch Perca fluviatilis) to investigate the effects of density dependence, predation, nutrient availability, climate and the timing of spawning on the cohort size distributions. We find that the mean length of the fish is best predicted by the extrinsic factors phosphorus concentration and summer temperature, and the densities of the different age‐classes, whereas the skewness of the length distribution is best predicted by phosphorus concentration, summer temperature, abundance of small fish, and the timing of spawning. Higher nutrient levels, temperatures and densities of small fish increase food availability and thus reduce competition, which is reflected in increased mean length and decreased skewness. The timing of spawning affects skewness presumably through changes in the initial size variation of the cohort and the length of the first growth season. Our results indicate that higher temperatures increase the mean length and decrease skewness due to the concurrent eutrophication of the lake. The study thereby highlights the potential impact of human‐induced environmental change on the size structure of fish populations. More studies are needed to understand better the complex mechanisms through which these factors alter the intensity of intraspecific competition in fish communities.

The effectiveness of different meso-scale rugosity metrics for predicting intra-habitat variation in coral-reef fish assemblages

Habitat structure is frequently an important variable affecting species’ abundances and diversity, and identifying the key aspects and spatial scales of habitat complexity is critical for understanding the ecology and conservation of a range of communities. Many coral-reef fishes are intimately linked with benthic habitat structure, and previous research has demonstrated rugosity as an important predictive variable of assemblage parameters. However, these studies typically consider rugosity at small scales, amalgamate multiple habitat features, or are semi-quantitative. This study considers meso-scale rugosity (within 51 plots of 25 m2 on a Belizean forereef) generated by varying coral densities, heights, and complexities. Seven rugosity metrics were calculated for each plot, and were regressed against each of 11 fish assemblage parameters. Intra-habitat variability of each fish parameter was significantly positively or curvilinearly correlated to at least one metric of meso-scale rugosity, but the metric generating the strongest correlation varied. The abundance of small fishes, and consequently most of the assemblage statistics (e.g., total fish abundance and diversity) were best predicted by the number of tall (>50 cm) corals. The abundance of damselfishes, parrotfish biomass, the abundances of medium-sized and large fishes, and total fish biomass were curvilinearly related to mean coral height. The abundances of wrasses and surgeonfishes were most strongly correlated with the number of corals within a plot. Because coral-generated meso-scale rugosity is an important factor influencing intra-habitat variation in fishes, it should be explicitly considered when investigating fish-habitat relationships and predicting the impacts of coral mortality on ecosystem processes and services.

Predicting patterns of richness, occurrence and abundance of small fish in New Zealand estuaries

Estuarine fish habitats are vulnerable to human impacts and are poorly studied. We surveyed 69 of New Zealand’s 443 estuaries across 1500 km to: determine species composition of small fishes; model and predict their richness, occurrence and abundance; test marine classification schemes as a basis for Marine Protected Areas; and inform impact mitigation measures. Boosted regression tree models produced acceptable fits for richness and occurrence at estuary and site scales and abundance at the site scale. Richness was greatest in northern North Island; the best predictors were estuary area and area of intertidal habitat. Within estuaries, richness increased towards the head, as water clarity declined and the substratum became muddier. Air temperature, estuary and intertidal area, tidal range and freshwater and seawater influx were the best predictors of occurrence at the estuary scale; water temperature and salinity were important at the site scale. Biological classification schemes seldom improved model fits and have little predictive utility. Richness predictions were made for 380 estuaries and occurrence predictions for 16 species. These predictions inform resource managers about estuarine fishes within their jurisdiction, bypassing the need to undertake expensive field surveys. However, sampling of environmental predictors is still required to drive some models.

The Stream-Lake Ecotone: Potential Habitat for Juvenile Endangered June Suckers (Chasmistes Liorus)

Potamodromous fish are poorly studied even though they are threatened often by human activities. The June sucker (Chasmistes liorus) is an endangered potamodromous species endemic to Utah Lake. Larval June suckers have not been collected from Utah Lake for at least 3 decades. Recruitment appears to be limited by low temperatures and scarce food, resulting in mass starvation of larval June suckers in the stream environment. We compared water temperature, Zooplankton food availability, and small fish abundance in the stream and in 3 habitats along the stream—lake ecotone (dense emergent vegetation, sparse emergent vegetation, and open lake) to test the hypothesis that all 3 factors would reach a maximum in the dense emergent vegetation of the stream—lake ecotone. We used the abundance of fathead minnows in each habitat type as a surrogate for small fish like juvenile June suckers. We found that temperature, food, and fathead minnows reached their maximums in the open lake rather than in vegetated habitats of the streamlake ecotone. The stream had the lowest average temperatures (15.1 °C) and the lowest Zooplankton concentrations (61 · L-1) over the growing season. Contrary to expectations, low temperatures (16.9 °C) and low food abundance (505 · L-1) also characterized the densely vegetated habitat, whereas the open lake had the highest temperatures (20.4° C) and highest concentrations of Zooplankton (2353 · L-1). Restoration should include a mechanism to transport larval fish through the densely vegetated portion of the stream—lake ecotone, which can be hundreds of meters wide, to the warm productive waters of the open lake. The braided planform of the terminal reaches of Hobble Creek should be replaced with shallow riffles to increase mean stream velocity and decrease the transport time of larval June suckers.

Spatial distribution and abundance of small fishes in Xiaosihai Lake, a shallow lake along the Changjiang (Yangtze) River, China

Spatial distribution and abundance of small fishes were studied in autumn 2007 in the Xiaosihai Lake, a shallow lake along the middle reach of the Changjiang (Yangtze) River. Based on the plant cover, the lake was divided into three major habitats: Myriophyllum spicatum habitat (MS habitat), Trapa bispinosa habitat (TB habitat), and non-vegetation habitat (NV habitat). A modified pop-net was used for quantitative sampling of small fishes in the three habitats, and the Zippin’s removal method was used for estimating densities of the small fishes. A total of 13 species belonging to 5 families were collected, with 11 species in MS habitat, 7 species in TB habitat, and 5 species in NV habitat. Habitat type had significant effect on the spatial distribution of small fishes. The Shannon-Wiener diversity index in the MS, TB and NV habitats were 1.28, 0.56 and 0.54, respectively. The total density and biomass of small fishes were significantly higher in the MS habitat (13.68 ind/m2 and 4.44 g/m2) than in the TB habitat (1.41 ind/m2 and 0.54 g/m2) and the NV habitat (1.08 ind/m2 and 0.40 g/m2), and were not significantly different between the TB habitat and the NV habitat. Water depth had no significant effect on spatial distribution of the small fishes. It was suggested that vegetation type played an important role in habitat selectivity of small fishes, and the presence of submersed vegetation should be of significance in the conservation of small fish diversity.

Spatial distribution of intertidal fishes: a pattern dependent on body size and predation risk?

Traditionally, heterogeneity levels have been associated with refuge availability and, consequently, with security areas for prey. Hence, there is assumed to be a positive correlation between the spatial distribution of potential victims and habitat heterogeneity levels. Among fish, the magnitude of predator-prey interaction has been associated with their size ratio, which includes body size as a central variable for individual predation risk. To date, the importance of body size and its interaction with spatial heterogeneity in relation to fish spatial distribution has been scarcely considered, being this the main objective of the present study. Based on a semi-experimental design, defined by the level of spatial heterogeneity, censuses in 24 artificial pools were conducted for 2 years at the rocky intertidal zone. This study focused on carnivorous fish assemblages residing off the rocky shore of central Chile. The results indicated a counterintuitive response of fish to spatial heterogeneity, with a higher abundance of small fish in open areas. Apparently, this differential distribution could be related to the size ratio between prey and predator (size-dependent susceptibility to predation), which is a hypothesis supported by decreases in the co-occurrence of fish with increasing size similarity.

Low nekton abundance in the western Bering Sea according to trawl survey data and model estimates

Complex trawl surveys were conducted in the upper epipelagic zone of the western Bering Sea and adjacent Pacific waters in the summer and fall seasons of 2002–2006. The abundance of small nekton (micronekton) was estimated using two independent methods: traditional trawling and a mathematical model of selective feeding by fish. According to the trawl data, total micronekton density varied from 1 to 158 (average 40) mg/m3 on the northwestern Bering Sea shelf and from 6 to 151 (37) mg/m3 in deep-water areas of the southwestern Bering Sea and adjacent Pacific waters. According to model calculations, micronekton density was higher—72–193 (141) mg/m3 on the shelf and 78–507 (228) mg/m3 in the deep-water part of the studied area. Both trawl and model data showed that small nekton on the northwestern shelf mostly consisted of larval and juvenile walleye pollock, as well as small fish species, such as capelin and Pacific sand lance. In the deepwater areas, mesopelagic fish and squid (northern lampfish, northern smoothtongue, and boreopacific gonate squid), which migrate to the surface at night, juvenile Atka mackerel, and shortarm gonate squid dominated among micronekton. The advantages and disadvantages of both the trawl and model methods for calculating the abundance of small fish and squid were considered. Comparison of abundance estimates for mass fish species, obtained through trawl and model methods, enabled us to analyze trawl catchability coefficients and propose a more differentiated division of micronekton into size classes than had been done earlier. A function that characterizes the dependence of the catchability coefficient (CC) on body length was offered for juvenile Atka mackerel. This equation can be also used for evaluation of CC for other fishes that have similar size and behavior.

Steeper biomass spectra of demersal fish communities after trawler exclusion in Sicily

AbstractSweeting, C. J., Badalamenti, F., D'Anna, G., Pipitone, C., and Polunin, N. V. C. 2009. Steeper biomass spectra of demersal fish communities after trawler exclusion in Sicily. – ICES Journal of Marine Science, 66: 195–202. The effects of trawling on Mediterranean demersal fish communities were assessed by comparing them with the normalized biomass spectra among three gulfs on the north Sicilian coast, one of which had been subject to 15 years of trawler exclusion. Comparisons were conducted across seasons and among depth strata. Biomass-spectra slopes were significantly steeper in the gulf that was closed to trawling than in the unprotected gulfs. This was attributed to the exclusion of trawlers, which lacked catch-size selectivity, and to the continued fishing by artisanal gears that are more size selective. The biomass of all size classes was higher in the protected gulf than in unprotected areas, and increases were greatest in smaller size classes. Community mean individual fish mass was similar among all areas with a wider range of body masses present in the trawl exclusion area, compensating for the greater abundance of small fish. Size-spectra slopes were generally shallower and midpoint heights lower with increasing depth and were greater in autumn than spring; the effect of these seasonal and depth factors was as great as that of protection. Depth patterns are explainable by bathymetric trends in within- and among-species fish size. Seasonal differences were attributed to variation in spawning behaviour and fishery recruitment, with seasonal differences being greater in unprotected locations as a result of recruitment overfishing.

Impacts of environmental change and direct and indirect harvesting effects on the dynamics of a marine fish community

We describe dramatic shifts in the species composition of the marine fish community of the southern Gulf of St. Lawrence using a 35-year time series of catch rates in an annual bottom-trawl survey. We attempt to understand the causes of these changes using a traits-based approach that relates the similarity among species in their abundance trends to similarities in their ecological traits. We selected traits based on a priori beliefs of how each should reflect susceptibility to changes in a different external factor potentially affecting the community. We found evidence for an effect of ocean climate and top-down effects of fishing and seal predation, but not for bottom-up effects of prey availability on adult fishes. Mean body length in the community decreased dramatically in the 1990s. This reflected the removal of large-bodied fishes by fishing and sharp increases in the abundance of small fishes. The biomass of small fish was inversely correlated with an index of predation on those fish by larger fish, suggesting strong predator control of the abundance of small-bodied fishes. Our results suggest that changes in ocean climate combined with direct and indirect effects of harvesting can dramatically and rapidly alter the composition of marine fish communities.