Size-specific social interactions and foraging styles in a shallow water population of mutton snapper,Lutjanus analis (Pisces: Lutjanidae), in the central Bahamas

Synopsis Field observations quantified the effects of fish size and time of day on activity patterns, intraspecific encounters, and foraging styles in mutton snapper,Lutjanus analis, during the spring and winter of 1991. Fish ranged in size from 15 to 65 cm fork length (FL), and were associated with an artificial patch reef system located on a shallow seagrass meadow in the Exuma Cays, Bahamas. The most common, non-resting diurnal activities were intraspecific chasing and displacing, and feeding. Intraspecific displacing was significantly higher during midday compared to morning or evening. The highest proportion of intraspecific encounters (combined chasing and displacing events) occurred among medium (25–35 cm FL) and large (> 35 cm FL) fish. The few large fish observed (13% of population) initiated the same proportion of encounters as the predominant (50%) medium fish. The remaining (37%) small fish (> 25 cm FL) were the least aggressive. Dark barred and dark nape color patterns were associated with displacing and chasing, respectively. Fish exhibited considerable variability in feeding behavior, Proportionally fewer fish fed during midday compared to morning or evening, although small fish fed proportionally more often than medium or large fish despite time of day. Picking was the primary feeding mode and was observed during all times of day. Winnowing was observed during midday and evening, whereas midwater strikes were confined to morning and evening. Small fish displayed proportionally higher picking and midwater strikes during morning and evening, respectively, compared to medium or large fish. However, large fish winnowed proportionally more often than small or medium fish during evening. Dark barred color patterns were associated with feeding on the substrate, whereas no color changes occurred during midwater strikes. Our results indicate thatL. analis forms dominance hierarchies and that high variability in foraging styles, according to fish size and time of day, may be a means to reduce intraspecific competition.     

The allometry of energy reserve depletion: test of a mechanism for size-dependent winter mortality

We experimentally tested the hypothesis that energy reserve depletion varies inversely with size in the fish Menidia menidia, an estuarine fish known to exhibit size-dependent winter mortality. Individuals in two size groups were starved at two winter temperatures (4°and 8°C) and sacrificed at a range of time intervals (up to 127 days). Lipid levels and lean tissue were analyzed to estimate somatic energy storage. As predicted, energy depletion was greater at high temperatures, and proportionally greater in small than in large fish. After 60 days of starvation at 4°C, small fish retained an average of 67% of their original energy reserves (vs 53% at 8°C), while large fish retained an average of 80% (vs 66% at 8°C). At 4°C, fish that were fed depleted their energy reserves as rapidly as unfed fish, but at 8°C, fish that were fed maintained reserves at higher levels than unfed fish. A high proportion of unfed fish (56% at 4°C, 27% at 8°C) died before they were to be sacrificed. Survival probability did not vary with size, nor was it influenced by the amount of energy reserves. The rate of energy depletion (equivalent to routine metabolic rate) decreased gradually over time, particularly in small fish. Routine metabolism did not conform to a single scaling relationship. Within each temperature-size group, the routine rate declined more rapidly than metabolically active mass (lean mass). At 8°C, the difference between size groups in energy depletion rate conformed closely to the expected allometry exponent of 0.8. In contrast, at 4°C, the estimated allometry exponent increased over the experiment (−0.19 to 2.5). We conclude that strategies to minimize energy loss may often modify bioenergetic scaling relationships. Received: 30 September 1998 / Accepted: 10 February 1999     

The establishment and subsequent history of a population of Ligula intestinalis in roach Rutilis rutilis (L.)

The introduction, establishment and subsequent history of a population of Ligula intestinalis in roach, Rutilus rutilus , of a small lake was studied over a period of seven years. The introduction is believed to be a natural, chance colonization achieved by the return of Great Crested Grebes as permanent residents. Within two years the parasite had infected 33% of the roach between 60–120 mm in size. It exhibited a pronounced seasonal cycle in infection. Fish were often infected when only a few months old in autumn, and young fish and parasites over-wintered together. Parasite growth resumed in the following summer, when some further infections occurred. The majority of infected fish died in or before their second winter, possibly as a result of selective predation, but a small proportion survived into a third year. Dispersion of the parasite was close to random for most of the time. Multiple infections and slight over-dispersion occurred only in summer months. Infections were scarcer in large fish, due partly to selective mortality of infected fish and partly to a change in the host diet. The pattern of seasonal infection compared favourably with other localities, but prevalence and intensity of infection were consistently lower, parasite dispersion closer to random and multiple infections much scarcer than elsewhere. Infection levels and parasite growth rates varied from year to year, but these annual fluctuations were irregular and showed no consistent pattern. The Ligula population exhibited both regulatory and destabilizing features and there was no definite evidence that it was stable. It is suggested that Ligula is an opportunist species. It is endemic in some sites but in others, especially small lakes, the populations are unregulated and unstable and so persist for short periods only in a non-equilibrium state.     

Relationship between body size and habitat complexity preference in age-0 and -1 year winter flounder Pseudopleuronectes americanus

The interaction between body size, habitat complexity and interstice width on habitat preference of age-0 and -1 year Pseudopleuronectes americanus was examined using continuous remote video observation. The habitat choices of juvenile P. americanus were recorded over a 6 h period in tanks with four treatments: bare sand, sand with low complexity cobble, sand with intermediate complexity cobble and sand with high complexity cobble. Both age-0 and -1 year fish preferred cobble to bare sand. Within cobble treatments, age-0 year fish preferred intermediate complexity cobble, with a 1.59 ratio of interstitial space to body width. The largest age-1 year fish (123-130 mm standard length, L(S) ) preferred low complexity cobble. While a significant preference was not detected, medium age-1 year fish (83-88 mm L(S) ) tended to select low complexity cobble, whereas small age-1 year fish (73-82 mm L(S) ) tended to select low and intermediate cobble, with an interstitial space to body width ratio of 1.05. For medium and large age-1 year fish, there was an increased selection of low complexity cobble, corresponding to larger interstitial space to body size ratios. This study indicates that juvenile P. americanus prefer complex habitat to unstructured habitat and that this preference is mediated by a relationship between fish body size and the size of structure interstices. These results contribute to the growing body of knowledge of complex habitat selection and drivers of habitat choice in flatfishes.     

School size affects individual feeding success in three-spined sticklebacks (Gastevosteus aculeatus L.)

The effect of school size on the feeding success of individual three-spined sticklebacks was studied. We found that the proportion of fish feeding on benthic prey increased with school size and that fish in large schools tended to start feeding sooner than fish in small schools. The total number of strikes also increased in larger schools. Despite this evidence for a foraging benefit associated with school membership we propose that improved feeding returns do not alone explain stickleback schooling.     

How feeding performance and energy intake change with a small increase in the body size of the three-spined stickleback

Changes in the foraging behaviour due to variation in the body size of the three-spined stickleback Gasterosteus aculeatus were investigated. All sizes of fish had a high probability of attacking prey whenever encountered. The probability of eating the prey increased with the size of the fish, as the larger fish had larger jaws and a greater stomach capacity. Therefore, as fish increased in size there was an increase in the probability of successful prey capture. The level of satiation did not have an effect on the prey handling time, which is contrary to other studies and is probably a result of the large prey sizes. The physical size of the prey meant that the handling times were long regardless of the motivational level of the fish. The larger fish took in more energy and at a faster rate, although the time to reach satiation was similar for all fish sizes. The advantage that large fish appear to have in successfully gaining large prey is negated by their greater metabolic requirement. The changes in feeding performance induced by small increases in body size could have important consequences for intraspecific competition, habitat Use and risk of predation.     

Fishing‐down within populations harms seed dispersal mutualism

Large fish are often the most effective seed dispersers, but they are also the preferred target for fisheries. We recently started to comprehend the detrimental impacts of the extirpation of large frugivorous fish species on natural forest regeneration, but we lack a general understanding of how intraspecific size‐selective harvest affects fish–fruit mutualism. Our literature review demonstrated that large individuals within populations positively affect diverse aspects of seed dispersal, from consuming a higher diversity of seeds to enhancing germination. Furthermore, we filled a research gap by studying how individual size variations within two small frugivorous fish species (<16 cm) affect seed dispersal in flooded savannas. Even within small‐bodied species, large individuals swallow a higher number of intact seeds, but not necessarily a higher proportion. Overall, our results demonstrate the disproportional role of large‐bodied individuals as key seed dispersers in flooded habitats. Consequently, fishing‐down within both large‐ and small‐bodied species can negatively affect seed dispersal and natural regeneration in overfished wetlands.     

Experimental impacts of fish on small and large cladocerans under eutrophic conditions

The impacts of fish and nutrient enrichment on the biomass of small and large cladoceran species were studied in mesocosms under eutrophic conditions. Fish shifted the ratio between small and large species to favour small cladocerans compared to control mesocosms, while nutrient enrichment favoured large cladocerans. Both fish vital activity and nutrient enrichment increased total and blue-green algae concentrations. While chlorophyll concentrations increased in response to fish and nutrient enrichment, there were no significant effects of fish on total cladoceran biomass and the relative proportion of large species due to high variability among replicates within treatments. However, fish and nutrient enrichment affected average total cladoceran biomass and the biomass of large species compared to control conditions.     

Effects of neonate size and food concentration on the life history responses of a clone of the hybrid Daphnia hyalina × galeata to fish kairomones

1. The influence of fish kairomones on the life history parameters of a clone of the hybrid Daphnia galeata × hyalina was measured in laboratory experiments with small and large neonates at one food concentration. The influence of fish kairomones on neonates of the same size at two food concentrations was also investigated.
2. Neonate size (maternal investment per egg) changed the influence of fish kairomones on life history parameters. Daphnids that were small at birth showed greater differences between fish (kairomones present) and no-fish (kairomones absent) treatments for age at maturity and number of eggs in the first clutch than animals that were large at birth. On the other hand, animals that were large at birth differed more in size at maturity between fish and no-fish treatments than small neonates.
3. The food concentrations used in the experiments (0.25–0.75 mg C l^–1) hardly affected the influence of fish kairomones on the life history characteristics studied.
4. The effect of fish kairomones on the size of neonates from the first clutch was similar for large and small daphnids. In the no-fish treatment significant differences were found between small and large neonates for age at maturity and number of eggs in the first clutch. In the fish treatment size at maturity differed significantly between small and large neonates.
5. In life history experiments, neonate size has to be taken into account when measuring the effects of fish kairomones because size at birth influences the results.     

Effects of neonate size and food concentration on the life history responses of a clone of the hybrid Daphnia hyalina × galeata to fish kairomones

1. The influence of fish kairomones on the life history parameters of a clone of the hybrid Daphnia galeata × hyalina was measured in laboratory experiments with small and large neonates at one food concentration. The influence of fish kairomones on neonates of the same size at two food concentrations was also investigated.
2. Neonate size (maternal investment per egg) changed the influence of fish kairomones on life history parameters. Daphnids that were small at birth showed greater differences between fish (kairomones present) and no-fish (kairomones absent) treatments for age at maturity and number of eggs in the first clutch than animals that were large at birth. On the other hand, animals that were large at birth differed more in size at maturity between fish and no-fish treatments than small neonates.
3. The food concentrations used in the experiments (0.25–0.75 mg C l^–1) hardly affected the influence of fish kairomones on the life history characteristics studied.
4. The effect of fish kairomones on the size of neonates from the first clutch was similar for large and small daphnids. In the no-fish treatment significant differences were found between small and large neonates for age at maturity and number of eggs in the first clutch. In the fish treatment size at maturity differed significantly between small and large neonates.
5. In life history experiments, neonate size has to be taken into account when measuring the effects of fish kairomones because size at birth influences the results.     

Spatial and trophic overlap of marked and unmarked Columbia River Basin spring Chinook salmon during early marine residence with implications for competition between hatchery and naturally produced fish

Ecological interactions between natural and hatchery juvenile salmon during their early marine residence, a time of high mortality, have received little attention. These interactions may negatively influence survival and hamper the ability of natural populations to recover. We examined the spatial distributions and size differences of both marked (hatchery) and unmarked (a high proportion of which are natural) juvenile Chinook salmon in the coastal waters of Oregon and Washington from May to June 1999–2009. We also explored potential trophic interactions and growth differences between unmarked and marked salmon. Overlap in spatial distribution between these groups was high, although catches of unmarked fish were low compared to those of marked hatchery salmon. Peak catches of hatchery fish occurred in May, while a prolonged migration of small unmarked salmon entered our study area toward the end of June. Hatchery salmon were consistently longer than unmarked Chinook salmon especially by June, but unmarked salmon had significantly greater body condition (based on length-weight residuals) for over half of the May sampling efforts. Both unmarked and marked fish ate similar types and amounts of prey for small (station) and large (month, year) scale comparisons, and feeding intensity and growth were not significantly different between the two groups. There were synchronous interannual fluctuations in catch, length, body condition, feeding intensity, and growth between unmarked and hatchery fish, suggesting that both groups were responding similarly to ocean conditions.     

The feeding ecology of freshwater fishes in two rivers of the Australian wet tropics

Synopsis The fish fauna of the Mulgrave and South Johnstone rivers is diverse relative to other Australian rivers and this study examines the diets of many of the resident fish species. Most species were small (< 200 g), and although considerable overlap in the size of the mouth was observed, closely related species tended to have non-overlapping ranges in mouth size. Five trophic guilds were recognised and substantial discrimination between guilds on the basis of body size was noted. Small fishes (< 5 gm body weight) consumed a variety of insect larvae and small terrestrial insects. The diet of large fish was characterised by the presence of large aquatic invertebrates and fish. A third group of intermediate sized fishes (10–20 gm), which included the 0+ age class of three species of large fish, also consumed aquatic invertebrates but only a small proportion of terrestrial invertebrates. Vegetable material was present in the diet of all three guilds but the source of that material varied. In groups 1 to 3 above, the source of that material was desmids and diatoms, aquatic macrophytes or filamentous alga respectively. A fourth guild fed predominately on detritus and some bivalve molluscs, and a fifth group, containing only two species, fed feavily on gastropod molluscs. The extent of dietary overlap varied both within- and between-rivers. In habitats dominated by low water velocities and sandy substrates, resource partitioning was pronounced and the number of fishes with empty or near empty guts was high, suggesting that food was more limited in this type of habitats. Little resource partitioning was observed in habitats characterised by a coarse substrate, high water velocity and dense riparian canopy.     

Male Reproductive Success in a Promiscuous Armoured Catfish Corydoras Aeneus (Callichthyidae)

Among a variety of fish mating systems, promiscuity with random-mating seems to be most prevalent. However, detailed studies of promiscuity have been rare due partly to the peculiar difficulty in examination of male mating and reproductive success in the random mating. Females of the armoured catfish Corydoras aeneus (no sexual dimorphism other than size of males > females) spawn 10–20 egg-clutches with multiple males at a time, but an entire egg clutch is inseminated by sperm of a single male. We studied mating system of this fish in aquarium. Males had neither mating territories nor monopolized females, never being aggressive against rival males. Evidence of female preference for certain male traits including size was not detected. Females mated a male in proportion to his relative courtship frequency among males. Courtship frequency was not related to male size, and male mating success was not different between small and large males. Clutch size and insemination rate were different neither between small and large males nor between frequently and less frequently courting males. Thus, the male reproductive success will not be related to the male size, but directly to courtship frequency, indicating the random mating in this fish. There seemed to be fecundity advantage with size in female, and the consequent sexual difference in energy allocation will be responsible to the sexual dimorphism. We also discuss the low male-GSI in this promiscuous fish in which sperm competition hardly occurred.     

Importance of ontogenetic habitat shifts to juvenile output and life history of Atlantic salmon in a large subarctic river: an approach based on analysis of scale characteristics

Juvenile and adult scale characteristics were used to compare two juvenile groups of Atlantic salmon in a large subarctic river in northern Scandinavia: individuals that have migrated from the main stem into small tributaries and those which remain in the main stem. Body size and scale measurements indicated enhanced growth in migratory parr as compared to their resident main stem counterparts. Analysis of adult salmon scale characteristics using maximum likelihood estimators revealed that 20% of the adults had been in the tributaries before the end of their second year of life, and more than 30% more had moved into the tributaries in the third year. Tributary fish matured at a smaller size and younger age (one-sea-winter salmon) than those rearing in the main stem which included a higher proportion of multi-sea-winter salmon. In addition, when smolt ages and ages at maturity were compared, older female smolts often resulted in smaller spawners and younger smolts, larger spawners. Small female spawners were more likely to survive to become repeat spawners.     

Silver spoons in the rough: can environmental enrichment improve survival of hatchery Atlantic salmon Salmo salar in the wild?

This study tested the ‘silver spoon’ hypothesis which posits that individuals that develop under favourable conditions should enjoy a fitness advantage later in life because they are more likely to recognize and settle in high‐quality habitats. Atlantic salmon Salmo salar of two age classes (0+ and 1+ years) were reared in environmentally enriched or standard hatchery tanks for a short period (c. 10 weeks), were then released into a natural river and sampled on repeated occasions to test for silver‐spoon effects. Compared with controls, enriched fish had a 6·4% higher recapture rate and settled in higher velocity habitats when they were stocked as 0+ year fry, but not when they were stocked as 1+ year parr. The opportunity for selection was generally higher for environmentally enriched fish than for controls, and also higher for 0+ than for 1+ year fish. Selection favoured individuals with high condition factor, extensive fat reserves and longer than average pectoral fins in both age classes but favoured a small body size in 1+ year and a large body size in 0+ year releases. Stomach analysis showed that enriched fish ate more, and adapted quicker to natural prey than controls. These results provide support for silver‐spoon effects in fish and indicate that enrichment can improve post‐release performance in conservation programmes, but seemingly only if fish are not kept in captivity for too long.     

Fish recruitment in a large,temperate floodplain: the importance of annual flooding,temperature and habitat complexity

1. Large river floodplains are considered key nursery habitats for many species of riverine fish. The lower Volga River floodplains (Russian Federation) are still relatively undisturbed, serving as a suitable model for studying the influence of flooding and temperature on fish recruitment in floodplain rivers. 2. We examined the interannual variability in recruitment success of young‐of‐the‐year (YOY) fish in the lower Volga floodplain in relation to flood pulse characteristics and rising water temperatures in the spring. We sampled four areas with different flooding regimes, in three consecutive years (2006–2008). 3. Extensive areas with a long duration of flooding accommodated high densities of young fish. This suggests that extended inundation improves the recruitment success of river fish. In areas with extensive flooding, the biomass of YOY of most fish species was about three times higher in 2006 and 2007 than in 2008. We hypothesise that low spring temperatures in 2008 may have caused this reduced recruitment and that a flood synchronised with rising temperature enhances recruitment success. 4. Extensive flooding was particularly favourable for species characterised by large body size, delayed maturation, high fecundity and low parental investment, such as pike Esox lucius, roach Rutilus rutilus and ide Leuciscus idus. Gibel carp Carassius gibelio, a species tolerant of high temperature and hypoxia, did particularly well in small waterbodies in the driest parts of the floodplain. 5. Structural characteristics of floodplain waterbodies explained much of YOY fish density. These species–environment associations varied from year to year, but some species such as common bream Abramis brama, roach and gibel carp showed consistent relationships with structural habitat characteristics in all years, despite large interannual fluctuations in flood pulse and spring temperature.     

Changes in reproductive strategy in the ruffe during a period of establishment in a new habitat

The pattern of maturation, body size and fecundity was examined in a population of ruffe ( Gymnocephalus cernuus L.) three times during a period of rapid growth, and eventual stabilization, following its introduction to a new habitat. When the ruffe were less common, maturing ruffe were relatively large and immature ruffe relatively small, compared with when the ruffe were abundant. Intermediate ruffe population size showed a maturation pattern intermediate between these two extremes. It is suggested that this pattern of maturation is a response of the ruffe population to changing growth opportunity induced by changing intraspecific competition. This fluctuating maturation pattern is interpreted in terms of a threshold-dependent maturation trigger, operating on the rate of accumulation of energy and a trade-off between somatic growth and gonad development. When the ruffe population was large, high intraspecific competition resulted in low opportunity for growth; only fish with the highest rate of food acquisition were able to mature in a given year–the investment in gonadal tissue reducing somatic growth. When the ruffe population was low, the high rate of energy acquisition in the population resulted in the triggering of maturation, even at small size, only the very smallest fish remaining immature. High growth opportunity allowed maturing fish to develop gonad and maintain somatic growth. The pattern of size related fecundity also changed over the three periods. When growth opportunity was low, size related fecundity was greater than when opportunity for growth was high. This suggests that maturing females faced with poor growth conditions compensated by increasing egg number for a given body size either by decreasing egg size or by increasing total investment in ovarian tissue.     

Life-history traits of invasive fish in small Mediterranean streams

We compared the life-history traits of native and invasive fish species from Catalan streams in order to identify the characters of successful invasive fish species. Most of the exotic fish species were characterized by large size, long longevity, late maturity, high fecundity, few spawnings per year, and short reproductive span, whereas Iberian native species exhibited predominantly the opposite suite of traits. Species native to the southeastern Pyrenees watershed were also significantly different from species native to the rest of the Iberian Peninsula but not native to this watershed. Iberian exotic species come predominantly from large river basins, whereas Catalan streams (and other small, coastal river basins) correspond to basins and streams of a smaller size and different hydrology, with differences in species composition and life-history traits of fish. The occurrence and spread of invasive species was not significantly related to life-history traits but to introduction date. The successful prediction of future invasive species is limited due to small differences in life-history and ecological traits between native and exotic species. Fecundity, age at maturity, water quality flexibility, tolerance to pollution and habitat seem the most discriminating life-history variables. This revised version was published online in July 2006 with corrections to the Cover Date.     

Shoaling behaviour of the two-spotted goby

Naturally formed shoals of adult Gobiusculus flavescens in a Swedish fjord ranged in size from a few individuals to several hundred fish and were sorted by body size. Shoal composition was highly dynamic and any particular group was unlikely to remain together for more than a few hours. Shoaling tendency of juveniles in laboratory experiments was high, and consistent preferences were demonstrated for numerically larger shoals. Large test fish preferred to associate with shoals composed of large, over shoals composed of small fish, whereas small test fish associated with both size classes equally. The ecological importance of shoaling in small shallow water fish is discussed, and possible mechanisms for the observed patterns are proposed.     

A Watershed Scale Assessment of Riparian Forests, with Implications for Restoration

A combination of air‐photo interpretation, field data, and Geographic Information System (GIS) analysis was used to map riparian areas that are likely to provide wood and shade to small‐ and medium‐sized streams and where, conversely, restoration might be most beneficial. The analysis encompassed all salmonid‐bearing waters of the Nooksack River basin, in northwest Washington State, plus small tributaries that were thought to contribute wood or effective shading. The size and composition of each riparian stand was examined to determine whether trees were large enough to contribute logs that would form pools in the adjacent channel, with pool‐forming size of wood a function of channel width. Riparian stands were classified according to whether they passed this pool‐forming test. Model results were an exact match to actual conditions in 69% of field‐verified stands. A large proportion (74%) of the stands failing the test in reaches of anadromous fish use were in agricultural areas. Passing stands typically had high shade levels, because both stream shade and effective large woody debris size are a function of the size of the trees relative to the size of the stream. The GIS layer of passing and failing riparian stands can be combined with layers depicting property ownership, threatened fish distribution, and other information to objectively prioritize riparian restoration locations and strategies.