Pelagic Fish Abundance Research Articles

Modeling the structure changes of cold-water copepods Calanus euxinus population under the influence of the black sea depths deoxygenation

The paper presents the results of modeling the possible changes in the structure of the population of Calanus euxinus Hulsemann, 1991 copepods in the Black Sea under the influence of deoxygenation that began in the late 1980s. Over the past 50 years, the depth of oxygen penetration in the deep sea has decreased by almost 50 m. The lower “life limit” of zooplankton in the Black Sea corresponds to the upper boundary of the suboxic layer, where intense copepod aggregation occurs at an oxygen concentration of 10 μM /L. In 1955–1976, this boundary was 130–140 m across the sea, but due to eutrophication and climate change since late 1980s, it has risen in deep water areas to 70–80 m. As a result, the dense copepod layers near the redoxcline and especially their reserve population stock, consisting of non-migrating individuals in diapause, may have been subjected to intensive eating by the Black Sea sprat, which lives to a depth of 100 m and can tolerate hypoxia. To explore this process, we created a new population dynamics model for C. euxinus, which makes it possible to assess changes in the abundance and structure of the population under conditions of a quasi-stationary state and during intensive predation of individuals in diapause. The model describes the dynamics of the Сalanus population based on a statistical description of the relationship between the growth of individuals, their fecundity, the duration of diapause, mortality, and environmental conditions: the concentration of food suspension and temperature in the layer of vertical migration. We put forward the hypothesis that the access of small pelagic fish to the concentration layers of copepods in diapause has a key effect on significant changes in the Black Sea ecosystem, associated with a decrease in the supply of forage plankton and fluctuations in the abundance of pelagic fish.

Unraveling the Complexity of the N e/N c Ratio for Conservation of Large and Widespread Pelagic Fish Species: Current Status and Challenges.

Estimating and understanding the ratio between effective population size (N e) and census population size (N c) are pivotal in the conservation of large marine pelagic fish species, including bony fish such as tunas and cartilaginous fish such as sharks, given the challenges associated with obtaining accurate estimates of their abundance. The difficulties inherent in capturing and monitoring these species in vast and dynamic marine environments often make direct estimation of their population size challenging. By focusing on N e, it is conceivable in certain cases to approximate census size once the N e/N c ratio is known, although this ratio can vary and does not always increase linearly, as it is influenced by various ecological and evolutionary factors. Thus, this ratio presents challenges and complexities in the context of pelagic species conservation. To delve deeper into these challenges, firstly, we recall the diverse types of effective population sizes, including contemporary and historical sizes, and their implications in conservation biology. Secondly, we outline current knowledge about the influence of life history traits on the N e/N c ratio in the light of examples drawn from large and abundant pelagic fish species. Despite efforts to document an increasing number of marine species using recent technologies and statistical methods, establishing general rules to predict N e/N c remains elusive, necessitating further research and investment. Finally, we recall statistical challenges in relating N e and N c emphasizing the necessity of aligning temporal and spatial scales. This last part discusses the roles of generation and reproductive cycle effective population sizes to predict genetic erosion and guiding management strategies. Collectively, these sections underscore the multifaceted nature of effective population size estimation, crucial for preserving genetic diversity and ensuring the long-term viability of populations. By navigating statistical and theoretical complexities, and addressing methodological challenges, scientists should be able to advance our understanding of the N e/N c ratio.

Life strategy of Antarctic silverfish promote large carbon export in Terra Nova Bay, Ross Sea

Antarctic silverfish Pleuragramma antarcticum is the most abundant pelagic fish in the High Antarctic shelf waters of the Southern Ocean, where it plays a pivotal role in the trophic web as the major link between lower and higher trophic levels. Despite the ecological importance of this species, knowledge about its role in the biogeochemical cycle is poor. We determine the seasonal contribution of Antarctic silverfish to carbon flux in terms of faeces and eggs, from samples collected in the Ross Sea. We find that eggs and faeces production generate a flux accounting for 41% of annual POC flux and that the variability of this flux is modulated by spawning strategy. This study shows the important role of this organism as a vector for carbon flux. Since Antarctic silverfish are strongly dependent on sea-ice, they might be especially sensitive to climatic changes. Our results suggest that a potential decrease in the biomass of this organism is likely to impact marine biogeochemical cycles, and this should be factored in when assessing Southern Ocean carbon budget.

The aggregation effect of offshore mussel farming on pelagic fishes

AbstractThe increase in hard artificial structures in the marine environment is contributing to ocean sprawl. As well as adding hard structure to the seabed, mussel aquaculture adds rope droppers and anchor lines to the water column, where such structure would otherwise be absent. This paper aims to monitor the effects of mussel headlines on the pelagic fish community over time, as well as to investigate whether there is an interaction between the age of cultivated mussels with the number of epibiota species and the abundance of pelagic fishes. Two surveys were carried out: a time series monitoring survey to assess whether the mussel headlines change the surrounding fish species relative to control areas over time, and a fish aggregation device effect survey to investigate whether older and larger mussels support greater abundances of pelagic fishes and epibiota. The most abundant fish species was Atlantic horse mackerel, Trachurus trachurus, with significantly greater numbers recorded around ropes growing older mussels. Results from the surveys show that the physical farm infrastructure has provided a substrate for colonisation by a variety of epibiota: A total of 21 species were recorded growing on the mussel ropes. Furthermore, the older and larger the mussels become, the greater the numbers of epibiota taxa that can colonise, coupled with a greater abundance of pelagic fish recorded around the ropes. The sustainable development of offshore Blue Industries is essential to produce renewable resources. This study brings essential evidence for marine managers and regulators that offshore mussel farming can provide benefits for pelagic and benthic species of both conservation and commercial importance while producing sustainable protein to feed a growing population.

Shelf oceanography and small pelagic fishes in the Java Sea

The optimization of small pelagic fisheries in the Java Sea could be enhanced by providing oceanographic information for analysis and determining potential fishing grounds. This study aims to investigate shelf oceanographic factors influencing small pelagic fisheries using multidatasets of sea surface temperature and surface salinity CROCO ocean model output (2010 – 2020), distribution of light-fishing vessel position of VIIRS satellite, and small pelagic fish catches landing data (round scads Decapterus spp. and the spotted sardinella Amblygaster sirm) at Pekalongan Fishing Port, Central Java. The results showed that the peak of fishing season for the spotted sardinella occurs during northwest monsoon, in which the eastward monsoon current and Sunda shelf throughflow brings fresher (±32.4 psu) and warmer (±28.5 oC) water into the Java Sea. In contrast, the peak of fishing season for round scads. is associated with saltier (>33.0 psu) and cooler (<28.5 °C) water supplied by the westward monsoon current and Makassar ITF during the southeast monsoon. Alternating intensification of monsoonal current along the northern Java and the southern Kalimantan coast during different monsoon periods is associated with a higher abundance of neritic and oceanic small pelagic fishes, respectively.

Assessing small pelagic fish trends in space and time using piscivore stomach contents

Changing distribution and abundance of small pelagic fishes may drive changes in predator distributions, affecting predator availability to fisheries and surveys. However, small pelagics are difficult to survey directly, so we developed a novel method of assessing the aggregate abundance of 21 small pelagic forage taxa via predator stomach contents. We used stomach contents collected from 22 piscivore species captured by multiple bottom trawl surveys within a vector autoregressive spatio-temporal model to assess trends of small pelagics on the Northeast US shelf. The goal was to develop a spatial “forage index” to inform survey and (or) fishery availability in the western North Atlantic bluefish ( Pomatomus saltatrix) stock assessment. This spatially resolved index compared favorably with more traditional design-based survey biomass indices for forage species well sampled by surveys. However, our stomach content-based index better represented smaller unmanaged forage species that surveys are not designed to capture. The stomach-based forage index helped explain bluefish availability to the recreational fishery for stock assessment and provided insight into pelagic forage trends throughout the regional ecosystem.

The potential of insects as alternative animal protein source for livestock feeding

The livestock industry as an important component of general agriculture is a key contributor to economic growth and development of any nation. In addition to having the capacity for earning revenue for the governments, it provides employment, food, farm energy, manure, fuel and transport. Currently, important protein ingredients for animal feed are fish meal, processed animal proteins and soybean meal. However, in the European Union the use of processed animal proteins in animal feed is prohibited due to the TSE legislation, globally land availability for soya cultivation is limited, while marine overexploitation has reduced the abundance of small pelagic forage fish from which fish meal and fish oil is derived. The growing scarcity of resources to produce these increasingly demanded ingredients has doubled prices during the last five years, while it already represents 60-70% of production costs. So, alternative (animal) protein sources for livestock and aquaculture are urgently needed.Insects are such an alternative animal protein source, which can sustainably reared on organic side streams. Reasons are that they have a favorable feed conversion efficiency, likely because they are cold-blooded. Insects contain between 30% and 70% protein on a dry matter basis. The protein content of the insect species is within the soybean/fish meal range and fat content is higher especially compared to (defatted) soybean meal. This review, however highlighted the environmental, health, livelihood and social benefits of insects. Recent efforts in feeding livestock, challenges limiting the use of insects as feed ingredients and moreover, the strategies for commercial production of insects as feed ingredients just to mention a few of the potentials of using insects for feeding livestock.

Mining previous acoustic surveys to improve walleye pollock (Gadus chalcogrammus) target strength estimates

Abstract Acoustic-trawl surveys are widely used to measure the abundance and distribution of pelagic fish. The echo integration method used in these surveys requires estimates of the target strength (TS, dB re 1 m2) of acoustic scatterers. Here, we present a new automated method to estimate TS from a large volume of previously collected acoustic survey data recorded near trawl sites. By applying a series of selection and filtering methods to echosounder data, single echo measurements representative of fish encountered during surveys can be objectively and reliably isolated from existing survey data. We applied this method to 30 surveys of walleye pollock (Gadus chalcogrammus) conducted in Alaska from 2007 to 2019 and estimated a new length-to-TS relationship. The resulting relationship ($TS = 20.0 \cdot {\log _{10}}\,L - 66.0$) was largely consistent with previous in situ estimates made during dedicated, mostly nighttime TS collection events. Analysis of this sizeable data set (n = 142) indicates that increased fish depth, lower ambient temperature, and summer months may increase pollock TS. The application of a new TS model incorporating these environmental covariates to historic surveys resulted in -16 to +21% changes in abundance relative to the model without environmental covariates. This study indicates that useful TS measurements can be uncovered from existing datasets.

Effects of Biotic and Abiotic Factors on the Spatiotemporal Distribution of Round Scad (Decapterus maruadsi) in the Hainan Island Offshore Area

Fishery resource and environmental data from four surveys conducted in the Hainan Island offshore area from March 2021 to June 2022 were used to investigate the spatiotemporal distribution of round scad (Decapterus maruadsi). A generalized additive model was applied to explore the relationships among abundance and distribution, also biotic and abiotic factors (i.e., temperature, salinity, chlorophyll-a concentration, water depth, phytoplankton abundance, zooplankton abundance, and jack mackerel (Trachurus japonicas) abundance). Round scad abundance (average 67.17 kg/km2) showed distinct spatial and seasonal differences around Hainan Island, with highest abundance in summer (171.72 kg/km2) and lowest abundance in spring (3.06 kg/km2). The optimal model revealed that jack mackerel abundance, sea bottom temperature, sea surface temperature, and latitude very significantly (p < 0.01) affected round scad distribution. Jack mackerel abundance (mainly in the range 0–50 kg/km2) was positively correlated with round scad distribution. The distribution showed a dome-shaped relationship with bottom water temperature in the range 18–30 °C, with maximum abundance at 24 °C. The distribution was negatively correlated with surface water temperature in the range 22–30 °C. Sea surface chlorophyll-a concentration and longitude were significantly correlated (p < 0.05) with round scad distribution. The results provide theoretical support to further investigate the formation mechanism of round scad fishing grounds and to enrich knowledge of pelagic fish abundance in the continental-shelf waters of the northern South China Sea.

Transport-driven seasonal abundance of pelagic fishes in the Chukchi Sea observed with seafloor-mounted echosounders

Abstract Recent summer surveys of the northeastern Chukchi Sea found pelagic fishes were dominated by large numbers of age-0 Arctic cod (Boreogadus saida, Gadidae) and walleye pollock (Gadus chalcogrammus, Gadidae), while adult fishes were comparatively scarce. The source and fate of these young fishes remain unclear, as sampling in this region is impeded by seasonal ice cover much of the year. Seafloor-mounted echosounders were deployed at three locations in the northeastern Chukchi Sea from 2017 to 2019 to determine the movement and seasonal variability of these age-0 gadids. These observations indicated that the abundance of pelagic fishes and community composition on the Chukchi Sea shelf were highly variable on seasonal time scales, with few fish present in winter. Tracking indicated that fish movements were strongly correlated with local currents. Fishes were primarily displaced to the northeast in summer and fall, with periodic reversals towards the southwest driven by changes in regional wind patterns. The flux of fishes past the moorings indicated that the prevailing northward currents transport a large proportion of the age-0 pelagic fishes present on the Chukchi shelf in summer to the northeast by fall, leading to relatively low abundances of age-1+fishes in this environment.

MENENTUKAN SWIMMING LAYER DAN DISTRIBUSI IKAN PELAGIS DI LAUT FLORES BAGIAN BARAT DENGAN DETEKSI AKUSTIK

The purpose of this study determined the swimming layer and distribution of pelagic fish in the waters west of the Flores Sea through acoustic detection. Experimental fishing is a method used with two data analyses: 1) analysis of acoustic data at sampling stations with stationery techniques; and 2) analysis of acoustic data along transects between stations. The detection results at stationery 1 showed a very low abundance of pelagic fish, distributed at layer 4 (150-200 m) at night and suspected to be a large pelagic type of tuna with a small amount. The detection results on stationery 2 and stationery 3 are relatively the same, showing a relatively higher abundance compared to stationery 1. Detection results for transects between stations 1-2, 2-3, 3-4, and 4-5 starting from Dewakang waters to east of Takarawataya waters did not detect schooling fish but only single fish. On transects between stations 5-6 very low abundance, the highest mean abundance of 0.5 fish/1000 m3 was detected at layer 4 (150-200 m). On transects between stations 6-7 the abundance was not too high, the highest mean abundance was detected at layer 3 of 10.8 fish/1000 m3. The transect between stations 7-8 is the highest abundance of all transects, the highest abundance average value detected at layer 3 is 20.3 fish/1000 m3. On transects between stations 8-9 the abundance was not too high, schooling tended to be present in all layers, and the highest mean abundance was detected at layer 4 of 1.3 fish/1000 m3. The results of the study concluded that the abundance of pelagic fish in the western Flores Sea is distributed in layers 3 and 4 (100-200 m).
 Keywords: Acoustic, distribution, pelagic fish, Flores Sea, layer

Climate-driven shifts in pelagic fish distributions in a rapidly changing Pacific Arctic

Baseline surveys of offshore pelagic fishes in the eastern Chukchi Sea in 2012 and 2013 found that age-0 Arctic cod (Boreogadus saida) dominated the pelagic fish community in summer, with relatively few adults present in the region. Since this time, drastic changes in the ocean-atmosphere-ice feedback loop have led to continued warming, further reducing ice cover, and increased northward transport has led to an increase in Pacific-origin waters on the Chukchi shelf in summer. To examine potential bottom-up effects of these environmental changes on pelagic fishes in this rapidly changing environment, we extended a time series of large-scale acoustic-trawl surveys with additional surveys in 2017 and 2019. Age-0 Arctic cod were the most abundant pelagic fish in all four survey years, comprising 68–93% of fish abundance. However, age-0 walleye pollock (Gadus chalcogrammus), which were scarce (<0.1% of fishes) and confined to the southern Chukchi in 2012 and 2013, were present in high abundance (>21% of fish abundance) throughout the Chukchi shelf in 2017 and 2019. Age-0 Arctic cod were substantially more abundant in 2017 than in other years, possibly due to increased survivorship of larvae under warm conditions. Unlike in 2017, Arctic cod and pollock were spatially separated in 2019 due to enhanced transport, with Arctic cod primarily present in the northeastern portion of the survey area, which was characterized by cool surface and bottom temperatures. The substantial increase in abundance of age-0 pollock in recent years suggests that environmental conditions now allow this species to extend its northern range into the southern and central Chukchi Sea, at least on a seasonal basis. The changes in abundance and species composition of pelagic fishes in the 2012–2019 time series are tightly coupled to recent changes in sea ice, temperature, and the increasing transport of Bering Sea waters through Bering Strait into the Chukchi Sea. Given that the environment is expected to experience further warming and increased transport, these northward shifts in species distribution are likely to persist in the future.

Modifying a pelagic trawl to better retain small Arctic fishes

Pelagic trawls are one of the primary methods of sampling midwater fishes. However, these trawls are species- and size-selective, and small fish can escape through trawl meshes. This can introduce uncertainty and bias into survey abundance estimates if not accounted for. The small, abundant pelagic fishes of the Alaska Arctic are challenging to sample with trawls as they are sufficiently motile to avoid small fine-mesh trawls but are also small enough to escape through the meshes of trawls designed to capture larger fishes. A pelagic herring trawl equipped with a fine-mesh codend liner was used to quantify the size and species composition of pelagic fishes during a baseline acoustic-trawl survey of the Chukchi Shelf. Subsequent experiments with recapture nets attached to the outside of the trawl netting suggested that escapement of small fishes was substantial, particularly in the aft net section. Thus, the trawl was further modified by reducing the taper in the aft net section and adding a small-mesh section in front of the codend to potentially reduce escapement. Further use of recapture nets during two subsequent acoustic-trawl surveys confirmed that this trawl modification substantially increased retention of small fishes and resulted in less size selectivity. These improvements will reduce biases in estimates of abundance, size, and species composition of pelagic Arctic fishes. This work highlights the importance of quantifying escapement from survey trawls and demonstrates that escapement estimates can guide successful trawl modifications.

EDNA assessment of pelagic fish diversity, distribution, and abundance in the central Pacific Ocean

In open ocean environment with rich biodiversity, the assessment of fish resources is critical to the development of marine ecosystems and the sustainable use of resources. Environmental DNA (eDNA) metabarcoding is a new, constantly evolving tool and has recently been used to examine deep-sea eukaryotic diversity and communities. Here, eDNA metabarcoding was used to monitor fish populations in the central Pacific Ocean to understand the distribution of marine biodiversity in the region and investigate the abundance of dominant species. Water samples in present study were collected from three habitats, a total of 73 fish species from 41 families were detected, of which, 15 fish species from 10 families were found to be dominant species. What is more, the dominant species: Elagatis bipinnulata, Encrasicholina punctifer, Euthynnus affinis, lisha elongata, Nomeus gronovii, Reinhardtius hippoglossoides, Vinciguerria nimbaria had significant differences between habitats (p<0.05 or p<0.01). Alpha diversity indicated that the fish community composition in the three habitats was generally high, which was the highest at the Equator habitat. Beta diversity based on Bray–Curtis and Jaccard indices indicated that the fish composition of the three habitats overlapped in part, but there were not significantly different (p >0.05). Regression analyses indicated that there was an overall correlation in the biomass and reads richness (p <0.05) of Thunnus obesus and Katsuwonus pelami This study not only verities the validity and accuracy of eDNA technology applied to pelagic fish biodiversity assessment but also provides systematic cases for fish resource surveys in other sea areas and lays the foundation for oceanic fish resource monitoring.

Seasonal oceanographic changes and their implications for the abundance of small pelagic fishes in the southern South China Sea

The seasonal variations in shelf circulation and their implications for the distribution of light fishing vessels positions (VBD) that capture small pelagic fishes in the southern South China Sea (SSCS) are investigated by using multi-datasets between 2008–2014. It is found that the SSCS shelf circulation is indicated by the strong seasonal reversal western boundary current (WBC) and prominent cyclonic circulation (eddy). The southeastward WBC brings homogeneous salty, colder, and high primary production of nanophytoplankton (PPN) water during winter-time from the northern and the cyclonic eddy region into the southern SSCS. In contrast, less salty, warmer, and less PPN are advected northwestward from Karimata into the northern region. These factors significantly influence the distribution of VBD and in turn the production of captured small pelagic fishes (CPUE), revealing two maxima (peaks) of both VBD and CPUE during the monsoon breaks (MB) with a time-lag of 2–3 month from the monsoon peaks. Relationship between oceanographic parameters and CPUE is examined statistically revealing that ocean current and PPN are significantly affect the CPUE. A relatively high PPN and strong current during the peaks of monsoon are correlated with lower CPUE. In contrast, during the MBs period CPUE maxima are associated with a relatively weak ocean current and consumed PPN that favor the abundance of small pelagic fishes.

Interactions between the common dolphin, Delphinus delphis, and the Portuguese purse seine fishery over a period of 15 years (2003–2018)

Abstract Interactions between cetaceans and the purse seine fishery in coastal waters of mainland Portugal were investigated using onboard observations over a period of 15 years (2003–2018). In 10% of the fishing sets, there were interactions with one of three species of cetaceans: common dolphin (Delphinus delphis), bottlenose dolphin (Tursiops truncatus), and harbour porpoise (Phocoena phocoena). The common dolphin was the most frequently observed species, occurring in 89% of all interaction events, and the only species with observed mortality. Therefore, this study focused only on the interactions with common dolphins. A generalized additive model (GAM) was applied to observer data to investigate how the fishing activity and the abundance of small pelagic fish influenced the probability of occurrence of interactions between the common dolphin and the purse seine fishery between 2003 and 2018. The results revealed significant annual and geographic variation in the occurrence of interactions, and a significant influence of fishing effort and catch per unit effort of sardine (Sardina pilchardus). A second GAM in a hurdle model was used to model the number of common dolphins that interacted with the fishing activity as a function of fishing seasonality and the local abundance of pelagic fish. There was a significant effect of the seasonality of the fishing activity and catch per unit effort of sardine and chub mackerel (Scomber colias) on the number of common dolphins interacting with the fishery. The results of this study suggest that the probability of interactions and the number of common dolphins interacting with the fishery were affected by the local abundance of sardine and chub mackerel. As the common dolphin population appears to be increasing in the study area, the risk of by‐catch also increases, thus the monitoring of the Portuguese purse seine fishery is important to inform the implementation of management plans, and also as a response to global efforts for sustainable fisheries and marine mammal conservation.

Seabird distribution is better predicted by abundance of prey than oceanography. A case study in the Gulf of Cadiz (SW, Iberian Peninsula)

AbstractQuantifying factors that influence marine predator distributions is essential to understanding the current and future change in marine biodiversity. Here, we test whether marine predator distribution relates to prey, or is better predicted by other specific habitat features. We examine the correlation between spatial distribution of three seabird species and their prey, as well as environmental proxies (oceanographic characteristics) in the Gulf of Cadiz, NE Atlantic. We modeled the at-sea distribution of Cory's shearwater, Balearic shearwater and Northern gannet, based on: (i) pelagic fish abundance according to acoustic surveys, and (ii) a forecast-model of remotely sensed environmental variables (productivity, sea surface temperature, and salinity). In general, seabird distributions were better predicted by abundance of fish than by environmental variables at the habitat scale. We obtained consistent correlations between seabird presence and the abundance of medium-sized (10–20 cm) sardines, anchovies and Mediterranean horse mackerel, providing information on their preferred prey. Additionally, oceanographic productivity variables moderately contributed to seabird distribution models, with better predictive value for the critically endangered Balearic shearwater and Northern gannet in the summer, whilst the model for Cory's shearwater's produced poorer predictions. Predator–prey combined studies may represent essential tools for an efficient ecosystem-based management of marine environments.

Data-mining previous surveys to improve walleye pollock target strength estimates

Acoustic-trawl surveys are widely used to measure the abundance and distribution of pelagic fishes. The echo integration method used in these surveys requires estimates of the target strength (TS, dB re 1 m2) of individuals to estimate abundance. In situ TS measurements are attractive to establish length-to-TS relationships because they are obtained under realistic conditions utilizing the same tools as those during surveys. However, these measurements are usually made in restricted circumstances, are limited in number, and data processing is often subjective. Here, we present a novel method to estimate TS from a large volume of previously collected survey data recorded at trawl sites. By applying a series of published filtering methods to TS data based on frequency response, packing density, and degree of co-location in overlapping beams, single fish echoes can be reliably isolated from existing survey data. The automated approach produced measurements representative of survey conditions (e.g., collected at survey speeds on free-swimming fish) from existing surveys. We applied this method to 30 surveys of walleye pollock (Gadus chalcogrammus) in Alaska and estimated a new length-to-TS relationship. The results were largely consistent with the relationship currently used for walleye pollock; depth and geographic area were significant covariates.

Changes in Pelagic Fish Community Composition, Abundance, and Biomass along a Productivity Gradient in Subtropical Lakes

How fish communities change with eutrophication in temperate lakes is well documented, while only a few studies are available from subtropical lakes. We investigate the fish community structure in 36 lakes located in the Yangtze River basin, covering a wide nutrient gradient. We found that fish species richness and total fish catch per unit effort (CPUE) increased significantly with chlorophyll a (Chla). Among the different feeding types, the proportion of zooplanktivores increased significantly with Chla, while the percentage of omnibenthivores showed no obvious changes; the CPUE of piscivorous Culter spp. increased with Chla, while their proportion of total catch decreased pronouncedly. Based on the index of relative importance (IRI), the most important and dominant fish species was the zooplanktivorous Sijiao (Toxabramis swinhonis), followed by the omniplanktivorous sharpbelly (Hemiculter leucisculus) and the omnibenthivorous crucian carp (Carassius carassius), a small-sized species belonging to the Cyprinidae family. The CPUE of these three species increased significantly with Chla. The focus has, so far, been directed at large fish, but as emphasized by our results, the abundant small fish species were dominant in our subtropical study lakes even in terms of biomass, and, accordingly, we recommend that more attention be paid to the population dynamics of these species in the future.

Stationary hydroacoustics demonstrates vessel avoidance biases during mobile hydroacoustic surveys of alewife in Lake Ontario

Mobile hydroacoustic surveys are routinely used to estimate pelagic fish abundance. In the Great Lakes, alewife are commonly surveyed with mobile hydroacoustics, however, their behavior often has them associated with epilimnetic habitats which increases the potential for vessel avoidance to bias hydroacoustic observations. Abundance estimates from mobile hydroacoustic surveys are typically made using depth and size thresholds to classify targets to species. In hydroacoustic surveys, fish can appear to be smaller when oriented off horizontal axis, as is common with fish displaying vessel avoidance behaviour. This presents a problem where alewives that are diving may appear too small to be correctly classified. We compared alewife swimming behaviours and target strength distributions between stationary up-looking and mobile down-looking hydroacoustics to quantify how vessel avoidance may bias hydroacoustic observations. Alewives from the mobile survey were found to be diving away from the surface, moving faster and in more linear paths than the undisturbed fish from the stationary survey. In the mobile survey, alewives were observed at smaller target strength values than would be expected if boat avoidance was not occurring. Our results suggest that alewife behavioral changes associated with vessel avoidance affect the number and size of targets classified as alewife from mobile hydroacoustic surveys in Lake Ontario.