Phenotypic Variability within and between Fish Shoals

Oecologia, 1998

Theory predicts that selection should favour phenotypic homogeneity in ®sh shoals, and ®eld studies have indeed con®rmed that variation in body length within ®sh shoals is signi®cantly lower than expected from a random distribution of ®sh among shoals. We investigated the extent to which variation in ®sh body length within shoals is determined by the shoal mean of body length, the number of species in a shoal, and the overall shoal size. We collected 34 ®sh shoals, ranging in size from 6 to 776 individuals, from the littoral zone of a Canadian lake. Shoals consisted of up to four dierent species, with multi-species shoals being larger and more frequent than single-species ones. The strongest determinant of body length variation within shoals was the shoal mean of body length, followed by the number of ®sh species in a shoal; i.e. multi-species shoals were less size-assorted than single-species ones. A more detailed analysis showed that the higher body length variation observed in multi-species shoals was due to increased body length variation both within and between component species. Shoal size had no signi®cant eect on body length variation within shoals. Potential explanations of the positive relationship between body length variation and the number of species in a shoal are suggested. The implications of the above results for the evolution of multi-species shoals are discussed.

1. The influence of body length and parasitism on the positioning behaviour of individuals in wild fish shoals was investigated by a novel means of capturing entire shoals of the banded killifish ( Fundulus diaphanus , Lesueur) using a grid-net that maintained the two-dimensional positions of individuals within shoals. 2. Fish in the front section of a shoal were larger than those in the rear. 3. Individuals parasitized by the digenean trematode ( Crassiphiala bulboglossa , Haitsma) showed a tendency to occupy the front of shoals. Parasitized fish were also found more in peripheral positions than central ones in a significant number of shoals. 4. Shoal geometry was affected by the overall parasite prevalence of shoal members; shoals with high parasite prevalence displayed increasingly phallanx-like shoal formations, whereas shoals with low prevalence were more elliptical.

Fish and Fisheries, 2003

Parasites & Vectors, 2013

Background: Parasite distribution is often highly heterogeneous, and intensity of infection depends, among other things, on how well hosts can avoid areas with a high concentration of parasites. We studied the role of fish behaviour in avoiding microhabitats with a high infection risk using Oncorhynchus mykiss and cercariae of Diplostomum pseudospathaceum as a model. Spatial distribution of parasites in experimental tanks was highly heterogeneous. We hypothesized that fish in groups are better at recognizing a parasitized area and avoiding it than solitary fish. Methods: Number of fish, either solitary or in groups of 5, was recorded in different compartments of a shuttle tank where fish could make a choice between areas with different risk of being infected. Intensity of infection was assessed and compared with the number of fish recorded in the compartment with parasites and level of fish motility. Results: Both solitary fish and fish in groups avoided parasitized areas, but fish in groups avoided it more strongly and thus acquired significantly fewer parasites than solitary fish. Prevalence of infection among grouped and solitary fish was 66 and 92 %, respectively, with the mean abundance two times higher in the solitary fish. Betweenindividual variation in the number of parasites per fish was higher in the "groups" treatment (across all individuals) than in the "solitary" treatment. Avoidance behaviour was more efficient when fish were allowed to explore the experimental arena prior to parasite exposure. High motility of fish was shown to increase the acquisition of D. pseudospathaceum. Conclusion: Fish in groups better avoided parasitized habitat, and acquired significantly fewer parasites than solitary fish. We suggest that fish in groups benefit from information about parasites gained from other members of a group. Grouping behaviour may be an efficient mechanism of parasite avoidance, together with individual behaviour and immune responses of fishes. Avoidance of habitats with a high parasite risk can be an important factor contributing to the evolution and maintenance of grouping behaviour in fish.

Biological Journal of The Linnean Society, 2020

The fragmented, heterogeneous and relatively depauperate ecosystems of recently glaciated lakes present contrasting ecological opportunities for resident fish. Across a species, local adaptation may induce diverse and distinct phenotypic responses to various selection pressures. We tested for intraspecific phenotypic structuring by population in a common native lake-dwelling fish species across a medium-scale geographic region with considerable variation in lake types. We investigated potential lake-characteristic drivers of trophic morphology. Using geometric morphometric techniques, we quantified the head shapes of 759 adult brown trout (Salmo trutta L.) from 28 lakes and reservoirs across Scotland. Multivariate statistical analyses showed that almost all populations differed from one another. Trout from larger and deeper lakes had deeper, but shorter heads, and smaller eyes. Higher elevation lakes were associated with fish with shorter heads and jaws. Within-population shape vari...

Ecological Monographs, 1998

We assessed the relative importance of environmental variation, interspecific competition for space, and predator abundance on assemblage structure and microhabitat use in a stream fish assemblage inhabiting Coweeta Creek, North Carolina, USA. Our study encompassed a 10-yr time span (1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992) and included some of the highest and lowest flows in the last 58 years. We collected 16 seasonal samples which included data on: (1) habitat availability (total and microhabitat) and microhabitat diversity, (2) assemblage structure (i.e., the number and abundances of species comprising a subset of the community), and (3) microhabitat use and overlap. We classified habitat availability data on the basis of year, season, and hydrologic period. Hydrologic period (i.e., pre-drought [PR], drought [D], and post-drought [PO]) represented the temporal location of a sample with respect to a four-year drought that occurred during the study. Hydrologic period explained a greater amount of variance in habitat availability data than either season or year. Total habitat availability was significantly greater during PO than in PR or D, although microhabitat diversity did not differ among either seasons or hydrologic periods. There were significantly fewer high-flow events (i.e., >2.1 mVs) during D than in either PR or PO periods. We observed a total of 16 species during our investigation, and the total number of species was significantly higher in D than in PR samples. Correlation analyses between the number of species present (total and abundant species) and environmental data yielded limited results, although the total number of species was inversely correlated with total habitat availability. A cluster analysis grouped assemblage structure samples by hydrologic period rather than season or year, supporting the contention that variation in annual flow had a strong impact on this assemblage. The drought had little effect on the numerical abundance of benthic species in this assemblage; however, a majority of water-column species increased in abundance. The increased abundances of water-column species may have been related to the decrease in high-flow events observed during the drought. Such high-flow events are known to cause mortality in stream fishes. Microhabitat use data showed that species belonged to one of three microhabitat guilds: benthic, lower water column, and mid water column. In general, species within the same guild did not exhibit statistically distinguishable patterns of microhabitat use, and most significant differences occurred between members of different guilds. However, lower water-column guild species frequently were not separable from all members of either benthic or mid-water-column species. Variations in the abundance of potential competitors or predators did not produce strong shifts in microhabitat use by assemblage members. Predators were present in the site in only 9 of 16 seasonal samples and never were abundant (maximum number observed per day was 2). In conclusion, our results demonstrate that variability in both mean and peak flows had a much stronger effect on the structure and use of spatial resources within this assemblage then either interspecific competition for space or predation. Consequently,' we suspect that the patterns in both assemblage structure and resource use displayed by fishes in Coweeta Creek arose from the interaction between environmental variation and speciesspecific evolutionary constraints on behavior, morphology, and physiology.

Ecological Monographs, 2014

Community assembly is affected by environmental filtering that restricts viable phenotypes and by species interactions that impose limits on interspecific trait similarity. Relative influences of these processes should vary according to habitat features and dispersal. Species dispersion within assemblage trait space also should vary in relation to species richness, strength of competition, and the spatiotemporal scale of analysis. We examined ecomorphological diversity of two freshwater fish families (Neotropical Cichlidae, Nearctic Centrarchidae) to test theories of local assembly from regional species pools and theories of species packing within mesohabitat patches. Cichlid and centrarchid assemblages were surveyed in four floodplain rivers (two in South America and two in North America) during low-water periods when fish densities are highest. Surveys were conducted in four mesohabitat types (submerged wood, leaf litter, rocks, sand bank) within river channels and floodplain lakes. We measured 23 morphological traits associated with locomotion and feeding. Principal components analysis was performed on the species 3 traits matrix, and species axis scores were used to calculate species pairwise Euclidean distances and indices of dispersion within assemblage morphospace: mean nearest-neighbor distance (indicating similarity), mean distance to centroid (assemblage morphospace size), and standard deviation of nearestneighbor distance (evenness of dispersion within assemblage morphospace). A null model was used to assess whether patterns were significantly nonrandom. When data for all mesohabitat types were combined for each river, species were significantly overdispersed and the assemblage morphospace was larger than predicted at random in every case. Analysis of assemblages within mesohabitat patches of different types revealed, in every case, significant overdispersion of species in morphospace indicative of limiting similarity. The total assemblage morphospace was greater than expected for tropical cichlids, but not for temperate centrarchids. Trends of species dispersion with assemblage morphospace in relation to species richness within mesohabitat patches were not consistent among or within river systems, possibly indicating that patches were already saturated with these perciform fishes. Interregional comparisons suggest an influence from both adaptive diversification and environmental filtering at broad spatial scales. At the scale of mesohabitat patches in lowland rivers, cichlid and centrarchid assemblages revealed patterns of trait complementarity that imply limiting similarity and strong influence of biotic interactions.

Oecologia, 1992

Species-habitat relationships are an important aspect of fish life history, particularly in early ontogeny.

Environmental Biology of Fishes, 1999

Although benthic insectivorous fishes such as darters and sculpins represent a significant component of riffle communities, few studies have compared the habitat use of these non-related but ecologically similar fishes. The objectives of this study were to examine the habitat use of Etheostoma olmstedi (tessellated darter) compared to Cottus bairdi (mottled sculpin) in Nescopeck Creek and Cottus cognatus (slimy sculpin) in Jack's Creek, Pennsylvania through underwater observation. Etheostoma olmstedi occupied habitats with significantly deeper waters than those available, whereas adult and young of the year Cottus occupied habitats with significantly faster water velocities than those available. Canonical discriminant analysis revealed microhabitat partitioning between E. olmstedi and each Cottus species. Cottus bairdi and C. cognatus occupied significantly shallower habitats with faster water velocities than E. olmstedi. Sculpin species were observed most frequently under substrate whereas E. olmstedi occurred most frequently on the top surface of the substrate. Hurlbert's standardized niche breadth values indicated that C. bairdi and C. cognatus were habitat specialists with regard to water velocity measures, but exhibited generalistic patterns of depth and substrate size use. Etheostoma olmstedi was a habitat specialist with respect to depth, but exhibited generalistic patterns of resource use for substrate size. Differential habitat use by these benthic fishes is consistent with the hypothesis that resource partitioning facilitates species coexistence among stream fishes.

The structure of social animal groups can be dynamic, characterized by high rates of group fission and fusion. Despite this, group composition is often well ordered by factors such as species, body size and by numerous other phenotypic traits. Research in shoaling fishes has revealed that individuals refine group membership decisions still further and are capable of assimilating chemical cues pertaining to recent habitat and prey use by prospective group mates, preferring to associate with others whose recent resource use history closely matches their own. In this study, we firstly examined the dynamics of the formation and breakdown of these preferences, revealing that they can be acquired and replaced in a matter of just a few hours. Using such cues enables individuals to accurately assess the resource use of conspecifics, allowing them to indirectly sample the local environment while reducing the chances of acquiring outdated information that can precipitate maladaptive behaviors. Secondly, we found that shoals composed of individuals with shared recent habitat use history were more cohesive compared to those where the constituent individuals differed in recent habitat use. Increased shoal cohesion may reduce predation risk, and could enhance the ability of individuals to detect and use social information.

Journal of Animal Ecology, 2008

Communities of different species are often structured according to niche differentiation associated with competitive interactions. We show that similar principles may apply on an ecological time-scale when individuals of a species having a wide size variation compete for resources, using the Lake Tanganyika cichlid Lobochilotes labiatus (5-30 cm). This species has a mouth especially adapted to suck up invertebrates from rock crevices. 2. Individuals defended feeding territories against similar-sized conspecifics, but not against differentsized ones. Thus, territories of similar-sized fish rarely overlapped, but up to a total of seven individuals (of seven size-ranks) had broadly overlapping territories with dissimilar-sized individuals. Comparison with expectation from the null model demonstrated clearly that observed size ratios between adjacent size rank were determined non-randomly regardless of sexual combinations. 3. Larger individuals took larger prey types of larger average size, but more importantly used wider rock crevices from which to suck food than smaller individuals. We calculated pairwise values of Schoener's index of diet overlap C d and the values of Levin's index of diet breadth B d (prey type and prey size) and the same for the width of the rock crevices used for foraging ( C r and B r ). C d remained high among all combinations of the seven ranks. In contrast, C r declined strongly in combinations of adjacent ranks (to 0·27), and was low or zero among further different size ranks. This shows that fish with overlapping territories divided the food resources largely through foraging site partitioning. Accordingly, B d did not depend on the size difference to the nearest two coinhabiting fish, whereas B r did. 4. We conclude that this L. labiatus community is structured non-randomly: body size-dependent effects on foraging site usage result in competition with, and territorial exclusion of, similar-sized individuals, but not of dissimilar-sized individuals that were accepted as coinhabitants. Accordingly, mean body size ratios (large/small) between two adjacent ranks were consistently approximately 1·28 [standard deviation (SD) = 0·07, n = 104], while approximately 1·34 from the null model (SD = 0·34, n = 10 400 simulations). We discuss our results as an example of Hutchinson's rule, applied originally to size ratios of different species.

Animal Behaviour, 2004

Journal of Fish Biology, 2001

Recruits of Lipophrys pholis, Coryphoblennius galerita and Gobius cobitis spend the low tide under water in pools, and in the case of G. cobitis also in channels. The pools used by the two blenniids show greater similarity than those used by G. cobitis. Larger juveniles and adults of C. galerita continue to use the same type of pools, which are temporarily abandoned by the males during the breeding season when they use emersed holes and crevices as nests. The much faster growing L. pholis and G. cobitis leave the pools in their first autumn, when they are c. 7 cm long. Whereas G. cobitis >7 cm move to channels and large upper pools, with a smooth substratum, sand and boulders, L. pholis begin to use emersed crevices where they spend low tide often in large groups. When L. pholis or C. galerita were added or removed selectively from pools, their numbers returned to pre-experimental levels after 2 weeks, suggesting that, although intraspecific competition may be strong, interspecific competition was unlikely to explain these results fully. 2001 The Fisheries Society of the British Isles

Behavioral Ecology and Sociobiology, 2006

Prior studies have shown that perch (Perca fluviatilis L.) of Lake Constance belong to two genetically different but sympatric populations, and that local aggregations of juveniles and adults contain closely related kin. In this study we analysed the genetic structure of pelagic perch larvae to investigate, if kin structured shoals already exist during early ontogenetic development or might be the result of homing to natal sites. Analysis of the gene frequencies at five microsatellite loci revealed that 3 out of 5 pelagic aggregations of larvae showed significant accumulation of kin. To investigate possible mechanisms of shoal formation, we tested if perch use olfactory cues to recognize their kin. Choice tests in a fluviarium showed preference for odours of unfamiliar kin versus unfamiliar non-kin. Additionally, we showed that perch could differentiate between the odours of the two sympatric populations and significantly preferred unfamiliar and unrelated conspecifics of their own over the foreign population. Our results present a behavioural mechanism that could lead to the observed formation of kin structured shoals in perch. We further discuss if the ability to discriminate between the own and the foreign population could result in assortative mating within populations and thus form the basis of "socially mediated speciation" in perch.

Global Ecology and Biogeography, 2002

North American freshwater fishes were studied to determine whether they displayed the same relationships between log (geographical range size) and log (body size) and the same pattern of range shape as found among North American birds and mammals. The forces that produce these patterns were also investigated. The log (geographical range size) : log (body size) relationship was analysed for 121 North American freshwater fish species. Thirty-two imperilled species were compared with 89 non-imperilled species to determine if the overall relationship could result from differential extinction. Range geometries were analysed, within and among habitat guilds, to determine if general patterns could be detected. The log (geographical range size) : log (body size) pattern among freshwater fish species was triangular and qualitatively similar to that found for North American birds and mammals. The results suggest that below a minimum geographical range, the likelihood of extinction increases dramatically for freshwater fishes and that this minimum range size increases with body size. The pattern of fish species' range shapes differs from that found for other North American vertebrate taxa because, on average, fish possess much smaller ranges than terrestrial species and most fish species' geographical ranges extend further on a north-south axis than on an east-west axis. The log (geographical range size) : log (body size) pattern reveals that fish species' geographical ranges are more constrained than those of terrestrial species. The triangular relationship may be caused by differential extinction of species with large bodies and small geographical ranges as well as higher speciation rates of small-bodied fish. The restricted geographical ranges of freshwater fishes gives them much in common with terrestrial species on oceanic islands. Range shape patterns within habitat guilds reflect guild-specific historical and current ecological forces. The overall pattern of range shapes emerges from the combination of ecologically different subunits.