Variation In Fish Density Research Articles

Impact of environmental enrichment and social group size in the aggressiveness and foraging activity of Serrapinnus notomelas

Fish are widely used in scientific research, and many species are bred for ornamental purposes and human consumption. Despite the large number of fish kept in captivity, research on the welfare of these animals has only recently intensified. One commonly used method for improving fish welfare is environmental enrichment, and the issue of animal density is central to this debate. Both enrichment and variation in fish density may have different impacts on the welfare of fish, which will depend on the biology of each species. In this study, we verified if environmental enrichment reduced aggressiveness and increased foraging activity in Serrapinnus notomelas in captivity. S. notomelas is a Neotropical characin commonly used in the aquarium trade. It has also been reared in captivity for scientific research purposes. However, studies on the welfare of S. notomelas in captivity are still lacking. We measured the aggressiveness and foraging activity of fishes in aquariums with different levels of enrichment (enriched and highly enriched), in comparison to an enrichment-free aquarium. The same behavioral aspects were quantified in groups of six and 12 individuals since it is known that group size influences these characteristics in shoal fishes. We found no influence of environmental enrichment on the species’ aggressiveness. However, fish housed in highly enriched aquariums increased their foraging activity to levels similar to fish housed in enrichment-free aquariums. We also found no influence of group size on the aggressiveness of individuals. However, increased group size had a positive effect on foraging activity, therefore we suggest housing S. notomelas with a minimum of 12 individuals. Results reinforce the need for species-specific investigation regarding both enrichment and shoal size to improve the welfare of captive fishes. We also encourage future studies on the effects of different enrichment levels, because we found that the level of enrichment affected the welfare of S. notomelas.

Accounting for spatiotemporal variation and fisher targeting when estimating abundance from multispecies fishery data

Estimating trends in abundance from fishery catch rates is one of the oldest endeavors in fisheries science. However, many jurisdictions do not analyze fishery catch rates due to concerns that these data confound changes in fishing behavior (adjustments in fishing location or gear operation) with trends in abundance. In response, we developed a spatial dynamic factor analysis (SDFA) model that decomposes covariation in multispecies catch rates into components representing spatial variation and fishing behavior. SDFA estimates spatiotemporal variation in fish density for multiple species and accounts for fisher behavior at large spatial scales (i.e., choice of fishing location) while controlling for fisher behavior at fine spatial scales (e.g., daily timing of fishing activity). We first use a multispecies simulation experiment to show that SDFA decreases bias in abundance indices relative to ignoring spatial adjustments and fishing tactics. We then present results for a case study involving petrale sole (Eopsetta jordani) in the California Current, for which SDFA estimates initially stable and then increasing abundance for the period 1986–2003, in accordance with fishery-independent survey and stock assessment estimates.

The effects of population density and lake characteristics on growth and size structure of brook trout Salvelinus fontinalis (Mitchill, 1815) in boreal forest lakes in Canada

This paper examines the effect of lake characteristics on population density and how this variation affects growth, mortality and population size structure of brook trout, Salvelinus fontinalis. The study was conducted on 17 recreationally fished, reproductively isolated boreal forest lakes in Newfoundland, Canada from 1993 to 2000. A standardized sampling program, the Fyke Littoral Index Netting program (FLIN) was used to collected data that describes brook trout population parameters and life history attributes. Regression analyses showed significant relationships between fish density and biomass and characteristics of the lakes. Variation in fish density and biomass was explained by lake surface area and littoral habitat area. Significant relationships were found when growth, mortality and size structure were regressed against density. The proportional stock distribution and theoretical maximum size of brook trout were negatively related to density, and natural mortality was positively related to density. The largest maximum length and highest proportional stock densities occurred at brook trout densities of less than 30 fish/ha. In general, the higher the proportion of littoral habitat area the higher the densities of brook trout, which correspondingly had important effects on growth, natural mortality and size structure of the brook trout populations. This information is critical to the development of management strategies aimed at altering size distribution to produce specific fisheries management outcomes.

Spatial Heterogeneity, Variable Rewards, Tag Loss, and Tagging Mortality Affect the Performance of Mark–Recapture Designs to Estimate Exploitation: an Example using Red Snapper in the Northern Gulf of Mexico

AbstractAccurate estimates of exploitation rate are essential to the management of exploited fisheries. Tagging studies are often used to estimate exploitation rates, but the performance of these approaches depends strongly on study design characteristics and the magnitude of assumption violations. We simulated a suite of candidate study designs for 1‐year high‐reward and variable‐reward tagging studies, exploring a range of sample sizes (number of tagged fish), exploitation rates, tagging mortality rates, tag loss rates, proportions of double‐tagged fish, and spatial variation in fish density, tag releases, and fishing effort. We calculated the uncertainty, biases, and reward costs of these candidate study designs to determine the most cost‐effective approach to accurately estimate exploitation rate for Red Snapper Lutjanus campechanus in Alabama waters of the Gulf of Mexico. We also investigated how incorrectly assuming a 100% reporting rate would affect these study results. Our simulations demonstrated that using all high‐reward tags provided more accurate and precise exploitation rate estimates than the variable‐reward approach but only if 100% reporting could be safely assumed. Further, distributing tags uniformly over the study area when the true spatial distribution of the population and fishing pressure varied over that area drastically biased exploitation rate estimates, suggesting that prior knowledge of the population's true spatial distribution over the study area is needed to ensure accurate estimates of exploitation rate. The most cost‐effective study design involved tagging between 400 and 1,600 fish with high‐reward tags, with 40% of the fish double‐tagged, and the tags spatially distributed in the same proportion as the population. However, violation of the 100% reporting rate assumption resulted in a proportional downward bias in the estimated exploitation rate. Simulation studies such as this are critical to ensure that cost‐effective study designs produce accurate and precise estimates of exploitation rate, particularly for high‐value species such as Red Snapper.Received July 13, 2016; accepted February 24, 2017 Published online April 14, 2017

Estimating Riverwide Abundance of Juvenile Fish Populations: How Much Sampling is Enough?

AbstractEstimating riverwide abundance of juvenile fish populations is challenging because detection probability is typically low and juveniles can be patchily distributed over large areas. We used a hierarchical Bayesian model to estimate the abundance of juvenile steelhead Oncorhynchus mykiss in two rivers in British Columbia over 3 years based on a multigear, two‐phase sampling design. These estimates were used to drive a simulation model to evaluate how the precision of abundance estimates varied with the number of single‐pass index and mark–recapture sites that were sampled, the proportion of shoreline sampled, and the mean and variation of detection probability and fish density across sites. The extent of variation in fish densities across index sites was the most important factor influencing the precision of river‐wide abundance estimates, and increasing the number of index sites was the best approach to reduce variability in abundance estimates. River size, which controls the proportion of habitat sampled for a given level of sampling effort, had a moderate effect on precision, but only when the extent of site‐to‐site variation in fish density was high. Factors affecting detection probability, such as the number of mark–recapture sites, the mean detection probability, or the extent of variation in detection probability across sites, had much less influence on precision of abundance estimates unless the proportion of river sampled was high. Hierarchical Bayesian models are no substitute for collecting informative data, but they improve our understanding of variance structure, which is critical for providing realistic estimates of uncertainty and designing informative and efficient sampling programs.Received July 16, 2015; accepted October 7, 2015 Published online March 8, 2016

Limited influence of a wind power project submarine cable on a Laurentian Great Lakes fish community

Summary Previous research has identified the generation of electromagnetic fields (EMFs) emanating from renewable energy project transmission cables to be a potential stressor to aquatic communities. In this study, we investigated whether the presence of a high voltage submarine transmission cable affected the spatial pattern and composition of nearshore and offshore fishes at a Laurentian Great Lakes site. The transmission cable investigated in this study runs 7.8 km along the lakebed of Lake Ontario, carrying electricity from the Wolfe Island wind power project to the city of Kingston, Ontario. In autumn of 2011, both nearshore electrofishing and deeper-water fisheries acoustic surveys were conducted along transects at varying distances to the cable. For both habitat types, no detectable effects of the cable on the fish community were found. Local habitat variables, including substrate or depth, were more important in explaining variation in fish density than proximity to the cable. Common species encountered during the surveys were round goby (Neogobius melanostomus) in the nearshore and alewife (Alosa pseudoharengus) in the deeper channel. American eel (Anguilla rostrata), thought to be an electromagnetically sensitive species, was also encountered during the surveys including in close proximity to the cable. More robust impact assessments require sampling fishes before a cable installation, over greater time frames (additional seasons or years), and habitats that support more diverse native assemblages.

Zooplankton response to climate warming: a mesocosm experiment at contrasting temperatures and nutrient levels

Zooplankton community response to the combined effects of nutrients and fish (hereafter N ? F) at contrasting temperatures was studied in a long-term experiment conducted in 24 shallow lake mesocosms with low and high nutrient levels. We found a positive effect of N ? F on zooplankton biomass, chlorophyll-a and turbidity. In contrast, zooplankton species and size diversity decreased with added N ? F, as did submerged macrophyte plant volume inhabited (PVI). The community composition of zooplankton in high N ? F mesocosms was related to chlorophyll-a and turbidity and to macrophyte PVI in the low N ? F mesocosms. Macrophytes can protect zooplankton from fish predation. Compared to N ? F effects, temperature appeared to have little effect on the zooplankton community. Yet analysis of community heterogeneity among treatments indicated a significant temperature effect at high N ? F levels. The results indicate an indirect temperature effect at high N ? F levels that can be attributed to tempera- ture-dependent variation in fish density and/or chlo- rophyll-a concentration.

Responses of stream fish populations to farming intensity and water abstraction in an agricultural catchment

Summary Agriculture affects streams worldwide, and multiple stressors are usually at work. The effects of farming intensification in the catchment are likely to interact with flow reduction due to abstraction for irrigation, but their combined effects on fish communities are unknown. In a low‐rainfall, agricultural catchment, we sampled instream physicochemistry and fish populations (non‐native brown trout, Salmo trutta, and native upland bullies, Gobiomorphus breviceps, dominated the fish communities) at 36 stream sites chosen to cover wide gradients of % Farming Intensity (%FI) (% catchment in ‘high‐producing exotic grassland’) and % Water Abstraction (%WA) (estimated from a published hydrological model). These landscape variables were not well correlated, allowing us to unravel their individual and combined effects. Presence of trout was best described by an additive multiple‐stressor model consisting of a unimodal response to %FI and a negative response to %WA. Trout density only showed a negative response to %FI. Upland bullies were unrelated to either landscape variable. When populations were modelled using instream variables, trout presence was negatively related to fine sediment depth, while density was negatively related to both sediment depth and total nitrogen (themselves closely related to %FI). Upland bully presence and density showed unimodal responses to just total nitrogen. Ammonium concentration was the only measured instream variable related to %WA. The final models for instream stressors explained more of the variation in fish density, whereas the final models for landscape stressors explained more of the variation in fish presence. Overall, farming intensity showed stronger negative relationships with fish populations than water abstraction, and fish were absent from stream reaches whose upstream catchments contained more than 40% high‐producing exotic grassland. Resource managers considering intensifying water abstraction or agriculture in low‐rainfall river catchments should be aware of the interplay between these two agricultural stressors.

Relative importance of habitat and fishing in influencing reef fish communities across seventeen Pacific Island Countries and Territories

AbstractReef fish assessments were undertaken in 17 Pacific islands to describe the status of finfish resources in 63 villages where fishing is mainly artisanal. Surveys were performed by recording the number and size of edible fish species and benthic composition. Fishing impact was described through six proxies representing level of catch, alternative incomes, degree of commercial catch and country economic development derived from a simultaneous socioeconomic assessment. The relative importance of broadly defined habitat (geographical location, island and reef type, substrate composition) and fishing impact in controlling the distribution of fish trophic groups, families and species was measured through multivariate analysis. The extreme faunistic diversity was shown by the large variation in fish density (difference of up to an order of magnitude) and fish biomass (displaying a 20‐fold difference across the region). Herbivores were dominant in the eastern part of the region, at what we classified as complex islands and at islands with small lagoon and at coastal reefs, while carnivores were dominant at oceanic islands and atolls and at outer reefs. Specific habitat associations were shown for Scaridae, Acanthuridae, Siganidae, Balistidae, Lethrinidae, Lutjanidae and Serranidae. Relative importance and size decrease of several fish families (Mullidae, Scaridae, Lutjanidae and Serranidae) were related to high fishing impact. Acanthuridae and Lethrinidae appeared to have a role as opportunistic groups in impacted sites. The relative impact from fishing and habitat on fishes accounted for, respectively, 20 and 30% of variance, demonstrating the effect of human impacts even at such large scale and taking into account only limited fishing impact variables.

Gender‐dependent effects of water quality and conspecific density on the feeding rate of fish – factors behind sexual growth dimorphism

Sexual growth dimorphism is common among animals, growth rate differing between the genders. Growth dimorphism is common also in fish, but the regulatory mechanisms remain unclear. Variations in feeding rate may lead to sexual growth dimorphism in fish, the growth rate of females decreasing steeper than the growth rate of males when feeding rate decreases. Because water quality strongly affects the prey detection by fish, variations in water quality could affect sexual growth dimorphism. Additionally, variations in fish density could affect dimorphism through food competition. We studied experimentally with perch Perca fluvialitis, whether the effects of decreasing water transparency and increasing fish density on the feeding rate of planktivorous fish are gender‐dependent. We expected that the feeding efficiency of females decrease steeper with increasing water colour and increasing fish density than the feeding rate of males. Additionally, we collected field data and studied the effects of water colour on the growth rate of male and female perch. The results showed that the effect of water colour on the feeding rate of perch was gender‐dependent, while perch density had no effect on the feeding rate difference between males and females. In highly humic water, the feeding rate of male and female perch did not differ, but in clear water females showed a significantly higher feeding rate than males. The results suggested that due to their high energy demand, female perch were feeding at high rate in both water colours, while the feeding rate of males in the clear water experiments was much lower than their possible maximum rate. This was probably due to the decreased feeding activity of males to reduce predation risk. The results were supported by field data, which revealed a significant effect of water colour on the gender growth difference in planktivorous 3‐year‐old perch. The results suggested that variations in water quality may be a factor behind the population‐dependency of dimorphism in fish.

Trajectories and magnitude of change in coral reef fish populations in Philippine marine reserves: a meta-analysis

Marine reserves are widely implemented worldwide to meet both conservation and fisheries management goals. This study examines the efficacy of Philippine marine reserves using meta-analysis by comparing variations in fish density (1) between reserves and adjacent fished reefs (spatial comparison), (2) within reserves before establishment relative to years following the establishment (temporal comparison), and (3) among reserves classified based on size, age, and enforcement capacity. A grand (total) mean of nineteen 22.3 ha coral reef reserves, protected for a mean duration of 8.2 years, were included in the meta-analyses. The overall density of fishes was higher in the reserves compared with the fished reefs and this difference was largely accounted for by exploited fishes. However, the overall density of fishes within the same reserves remained similar from the period before its establishment to several years following its establishment. Only the density of nonexploited fishes increased significantly during years subsequent to the establishment of the reserves. Neither age nor size of reserves correlated with pattern of change in fish density following the establishment of the reserves; however, fish density was consistently higher in larger and older reserves relative to smaller and younger reserves in the spatial comparison. Furthermore, well-enforced reserves had higher density of exploited fishes relative to less-enforced reserves in both spatial and temporal comparisons. In general, the magnitude and trajectory of change in fish density following the establishment of Philippine marine reserves are influenced by (1) functional groups of fishes under consideration, (2) size and age of the reserve, and (3) level of enforcement of the regulatory mechanisms necessary to sustain a marine reserve.

Population biology and habitat associations of benthic fish species in the shallow areas of a Mediterranean coastal lagoon (SE Iberian Peninsula)

The present study investigates the seasonal variation in fish abundance, fish biomass, reproductive status and population structure of four dominant benthic fish species, Salaria pavo, Pomatoschistus marmoratus, Gobius cobitis and Gobius niger, in the shallow and littoral areas of the Mar Menor coastal lagoon. In addition, the seasonal habitat associations of each species were studied by assessing environmental variables related to the habitat structure: submerged vegetation cover, submerged vegetation density, water depth and substrate composition. The temporal variations in fish density, standing stock and size frequency distributions of these species can largely be attributed to the seasonality of their breeding and juvenile recruitment periods. Moreover, habitat associations for these species were similar to those obtained in other Mediterranean and Atlantic coastal lagoons.

Environmental variability and population dynamics of juvenile brown trout (Salmo trutta) in an upstream and downstream reach of a small New Zealand river

Population densities, size and condition factor of juvenile brown trout and variation in environmental factors (discharge, temperature, and food availability) were examined over 2.5 years (28 months) in an upstream and downstream reach of Silverstream, a lowland river in New Zealand. Population densities of juvenile brown trout varied between the upstream and downstream reach, with the latter showing considerably higher temporal variation in fish density than the upstream reach. Juvenile brown trout from the downstream reach were larger and had higher condition factors than those from the upstream reach over the study period. Discharge had a negative effect on fish densities in 1 year in the downstream reach, and high water temperatures were recorded in one summer, with possible negative effects on fish densities. However, the different effects of environmental variables on populations between the two reaches could not explain the higher temporal variation in fish densities in the downstream reach as spatial variation in population densities also occurred in years without high discharge events or high summer water temperatures. Variability in food availability over time was higher in the upstream reach, but differences between the two reaches were small. These results suggest that factors other than condition of the local environment are important in controlling brown trout population dynamics in Silverstream.

Relationships between short-term variations in density of juvenile yellowfin goby Acanthogobius flavimanus and environmental variables on an estuarine mudflat

Weekly variations in density of juvenile yellowfin goby Acanthogobius flavimanus with a variety of environmental variables (e.g. water temperature, salinity and transparency, and prey density) were investigated on a tidal mudflat within the Tama River estuary, central Japan, from March to July 2001. Metamorphosing newly settled juveniles occurred from mid-March to late May. Metamorphosed benthich juveniles first appeared in late March, the density sharply increasing to a peak (67.0 ind./m2) in early May but rapidly decreasing to less than 10% of that two weeks later. No consistent relationships were apparent between short-term variations in fish density by developmental stages and water temperature, or salinity. In contrast, a weak negative relation was found between juvenile density and water transparency. Further, benthic juvenile density was positively related to short-term fluctuations of errant polychaetes, which is one of the main prey items.

Flatfish-habitat associations in Alaska nursery grounds: Use of continuous video records for multi-scale spatial analysis

Flatfish distributions have traditionally been described in terms of depth, temperature, and sediment characteristics, but other environmental variables may be important depending upon spatial scale. Surveys for age-0 northern rock sole ( Lepidopsetta polyxystra) were conducted in five near-shore nursery sites at Kodiak Island, Alaska, using a towed camera sled integrated with navigational data. The continuous record of fish density and habitat features made possible a spatially comprehensive analysis of fish-habitat associations at several spatial scales, ranging from tens of kilometres to less than 1 m. A combination of multivariate statistical interpretation and geographic information systems (GIS) revealed that the distribution of juvenile rock sole was associated with environmental variables and spatial scales that are not normally detectable with usual flatfish— and habitat—sampling methods (i.e., trawls and grabs). Generalized additive models (GAM) incorporating habitat variables determined from video provided large improvements over models using only the traditional variables such as depth and sediment type. At the broadest (regional) scale of analysis, combinations of sediment composition, surface bedform, temperature, and density of worm tubes provided the best model for rock sole density. Within-nursery variation in fish density was modelled best with depth, habitat structural complexity created by emergent fauna and macroalgae, and worm tube density. At the microhabitat scale (< 1 m), there was little evidence of direct contact between rock sole and structures such as shell or algae. Rather, they were loosely associated on a scale tens of metres. This study showed that spatially comprehensive surveys can be conducted with towed camera systems and without the need for sediment grab samples. This approach yields detailed habitat information for fishes and the opportunity for landscape analysis of spatial patterns that will be important in conserving critical habitats for flatfishes and other fish species.

Effect of the spatial arrangement of habitat patches on the development of fish habitat models in the littoral zone of a Canadian Shield lake

We developed fish habitat models in a Canadian Shield lake using (i) a sampling-site approach based on analytical units having a surface area equal to that of sampling sites (S ~ 200 m2), (ii) a constant-multiple approach in which the analytical units constituted grouping of adjacent sampling sites in units of increasing sizes (e.g., 2S or 3S), and (iii) a habitat-patch approach in which only contiguous sampling sites with similar environmental characteristics were merged. The best models explaining within-lake variations in fish density, biomass, and community structure on the littoral zone were obtained using the constant-multiple approach, but the predictive power of these models was highly variable (0 &lt; R2 &lt; 0.9) compared with the habitat-patch approach (0.27 &lt; R2 &lt; 0.49). For these approaches, intrinsic variables (estimated inside the analytical units) explained on average 16%–27% of the variations of fish descriptors compared with 6%–32% for extrinsic variables (observed outside analytical units or related to the spatial arrangement of habitat characteristics). Our study suggests that habitat patches are reliable analytical units with which to develop fish-habitat models. Our study also indicates that inclusion of variables that refer to landscape characteristics may significantly improve the predictive power of fish habitat models.

Use of GIS‐Derived Landscape‐Scale Habitat Features to Explain Spatial Patterns of Fish Density in Michigan Rivers

AbstractBoth site‐ and landscape‐scale processes play important roles in the biological communities of rivers. Understanding the influences of these processes on fish abundance can help direct management and research efforts toward appropriate habitat variables and scales. We used multiple linear regression analysis of a regional fish and habitat database to determine the feasibility of using geographical information systems (GIS)–derived landscape‐scale habitat variables to explain the spatial variation in the density of five sport fish species (Chinook salmon Oncorhynchus tshawytscha, steelhead O. mykiss, brown trout Salmo trutta, brook trout Salvelinus fontinalis, and white sucker Catostomus commersonii) in the rivers of Michigan's Lower Peninsula. We compared these models with those developed using site‐scale variables traditionally measured in the field. Landscape‐scale riverine habitat variables obtained through GIS analysis and modeling of catchment characteristics accounted for 18–69% of the variation in fish density. Landscape estimates of mean July water temperature were negatively correlated with the density of brook trout, brown trout, and Chinook salmon. Drainage area was negatively correlated with the density of steelhead and white suckers, and 90% exceedence flow yield (a measure of flow stability) was positively correlated with the density of Chinook salmon and steelhead. Site‐scale habitat variables explained less (12–57%) of the variation in fish density than landscape‐scale variables. In the site‐scale models, depth was negatively related to all species' densities, and the percentage of soft substrates was positively correlated only with white suckers. Although there was still much unexplained variation in density, our models provide insight into key habitat variables that influence fish density patterns on a large scale.

Variation in fish density, assemblage composition and relative rates of predation among mangrove, seagrass and coral reef habitats

We tested the hypothesis for several Caribbean reef fish species that there is no difference in nursery function among mangrove, seagrass and shallow reef habitat as measured by: (a) patterns of juvenile and adult density, (b) assemblage composition, and (c) relative predation rates. Results indicated that although some mangrove and seagrass sites showed characteristics of nursery habitats, this pattern was weak. While almost half of our mangrove and seagrass sites appeared to hold higher proportions of juvenile fish (all species pooled) than did reef sites, this pattern was significant in only two cases. In addition, only four of the six most abundant and commercially important species (Haemulon flavolineatum, Haemulon sciurus, Lutjanus apodus, Lutjanus mahogoni, Scarus iserti, and Sparisoma aurofrenatum) showed patterns of higher proportions of juvenile fish in mangrove and/or seagrass habitat(s) relative to coral reefs, and were limited to four of nine sites. Faunal similarity between reef and either mangrove or seagrass habitats was low, suggesting little, if any exchange between them. Finally, although relative risk of predation was lower in mangrove/seagrass than in reef habitats, variance in rates was substantial suggesting that not all mangrove/seagrass habitats function equivalently. Specifically, relative risk varied between morning and afternoon, and between sites of similar habitat, yet varied little, in some cases, between habitats (mangrove/seagrass vs. coral reefs). Consequently, our results caution against generalizations that all mangrove and seagrass habitats have nursery function.

Diurnal variations in the fish assemblage at an artificial reef

An important factor contributing to the variation in reef fish assemblages is the diel cycle of the constituting species. To study diurnal variations, visual censuses were carried out during day and night on a 5-year-old artificial reef in the coastal waters of the Algarve (southern Portugal). The reef consists of 735 small cubic modules (2.7 m 3 each) distributed in 21 groups. Counts were made according to a fixed time schedule (7:00, 8:30, 10:00, 11:30, 14:30, 19:00, 20:30, and 23:00 h) by scuba divers in August 1995. A total of 18 species were recorded, most of which belonged to the Sparidae. The number of species present on the reefs varied throughout the day, with minima during night and maxima around midday. The variation in fish density between day and night showed a similar trend. We conclude that most species present have a diurnal activity pattern. About 61% of the species were considered resident, while 33% used the reefs mainly for foraging and/or shelter. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

Influences of Spatial and Temporal Variation on Fish-Habitat Relationships Defined by Regression Quantiles

Abstract We used regression quantiles to model potentially limiting relationships between the standing crop of cutthroat trout Oncorhynchus clarki and measures of stream channel morphology. Regression quantile models indicated that variation in fish density was inversely related to the width:depth ratio of streams but not to stream width or depth alone. The spatial and temporal stability of model predictions were examined across years and streams, respectively. Variation in fish density with width:depth ratio (10th-90th regression quantiles) modeled for streams sampled in 1993-1997 predicted the variation observed in 1998-1999, indicating similar habitat relationships across years. Both linear and nonlinear models described the limiting relationships well, the latter performing slightly better. Although estimated relationships were transferable in time, results were strongly dependent on the influence of spatial variation in fish density among streams. Density changes with width:depth ratio in a single st...