Context‐specific influence of water temperature on brook trout growth rates in the field

1. Modelling the effects of climate change on freshwater fishes requires robust field‐based estimates accounting for interactions among multiple factors. 2. We used data from an 8‐year individual‐based study of a wild brook trout (Salvelinus fontinalis) population to test the influence of water temperature on season‐specific growth in the context of variation in other environmental (i.e. season, stream flow) or biotic factors (local brook trout biomass density and fish age and size) in West Brook, a third‐order stream in western Massachusetts, U.S.A. 3. Changes in ambient temperature influenced individual growth rates. In general, higher temperatures were associated with higher growth rates in winter and spring and lower growth rates in summer and autumn. However, the effect of temperature on growth was strongly context‐dependent, differing in both magnitude and direction as a function of season, stream flow and fish biomass density. 4. We found that stream flow and temperature had strong and complex interactive effects on trout growth. At the coldest temperatures (in winter), high stream flows were associated with reduced trout growth rates. During spring and autumn and in typical summers (when water temperatures were close to growth optima), higher flows were associated with increased growth rates. In addition, the effect of flow at a given temperature (the flow‐temperature interaction) differed among seasons. 5. Trout density negatively affected growth rate and had strong interactions with temperature in two of four seasons (i.e. spring and summer) with greater negative effects at high temperatures. 6. Our study provided robust, integrative field‐based estimates of the effects of temperature on growth rates for a species which serves as a model organism for cold‐water adapted ectotherms facing the consequences of environmental change. Results of the study strongly suggest that failure to derive season‐specific estimates, or to explicitly consider interactions with flow regime and fish density, will seriously compromise our ability to predict the effects of climate change on stream fish growth rates. Further, the concordance we found between empirical observations and likely energetic mechanisms suggests that our general results should be relevant at broader spatial and temporal scales.     

Using parentage analysis to estimate rates of straying and homing in Chinook salmon (Oncorhynchus tshawytscha)

We used parentage analysis based on microsatellite genotypes to measure rates of homing and straying of Chinook salmon (Oncorhynchus tshawytscha) among five major spawning tributaries within the Wenatchee River, Washington. On the basis of analysis of 2248 natural‐origin and 11594 hatchery‐origin fish, we estimated that the rate of homing to natal tributaries by natural‐origin fish ranged from 0% to 99% depending on the tributary. Hatchery‐origin fish released in one of the five tributaries homed to that tributary at a far lower rate than the natural‐origin fish (71% compared to 96%). For hatchery‐released fish, stray rates based on parentage analysis were consistent with rates estimated using physical tag recoveries. Stray rates among major spawning tributaries were generally higher than stray rates of tagged fish to areas outside of the Wenatchee River watershed. Within the Wenatchee watershed, rates of straying by natural‐origin fish were significantly affected by spawning tributary and by parental origin: progeny of naturally spawning hatchery‐produced fish strayed at significantly higher rates than progeny whose parents were themselves of natural origin. Notably, none of the 170 offspring that were products of mating by two natural‐origin fish strayed from their natal tributary. Indirect estimates of gene flow based on F_ST statistics were correlated with but higher than the estimates from the parentage data. Tributary‐specific estimates of effective population size were also correlated with the number of spawners in each tributary.     

Kinship analysis of brook trout Salvelinus fontinalis during their breeding migration

Microsatellite markers were used to test whether groups of pre‐spawning adult brook trout Salvelinus fontinalis from the same population and captured at the same location during their breeding migration comprised kin. Only weak evidence for kin associations was found at the onset of breeding: the proportion of kin captured at the same location was low and similar to the proportion found across all locations and the average relatedness of S. fontinalis captured at the same location was low. A dilution of kin associations from the feeding to breeding phase is hypothesized to stem from mainly natural mortality that reduces family size by the adult stage. The results illustrate the dynamic nature of kin associations between consecutive life stages, even within the same fish population.     

Diel activity patterns of the fish community in a temperate stream

The diel activity patterns of fishes in a temperate New Brunswick stream were studied during the summer over 5 years. Young‐of‐the year Atlantic salmon Salmo salar and blacknose dace Rhinichthys atratulus were more active during the day than at night, whereas lake chub Couesius plumbeus, brook trout Salvelinus fontinalis and adult white suckers Catostomus commersonii were more active at night than during the day. Because fishes were as likely to be nocturnal as diurnal, the data suggest that more night‐time sampling is needed to provide an unbiased view of fish community structure in temperate streams.     

The effect of stocking with 0+ year age‐class Atlantic salmon Salmo salar fry: a case study from the River Bush,Northern Ireland

An enhancement programme based on stocking 0+ year age‐class Atlantic salmon Salmo salar, conducted in the River Bush, Northern Ireland, U.K. over the period 1996–2005, was reviewed with reference to the performance and biological characteristics of wild fish. Wild ova to 0+ year fry (summer) survival was c. 8% with subsequent wild 0+ year fry‐to‐smolt survival c. 9%. Stocked unfed 0+ year juveniles gave c. 1% survival to smolt whilst fed 0+ year S. salar stocked in late summer exhibited survival at c. 5%. Stocking with unfed and fed fry contributed to increased smolt production and helped attain local management objectives between 2001 and 2005. Significant differences in biological characteristics were observed between wild and stocked‐origin fish. Wild‐smolt cohorts were dominated by 2+ year age‐class fish on the River Bush whilst smolts originating from fed fry mostly comprised younger 1+ year individuals. The mean mass of 1+ year smolts derived from stocked fed fry was significantly lower than that of wild 1+ year smolts, although these differences were not evident between older age classes. Differences in run timing between wild smolts and smolts derived from stocked fry were also apparent with the stocked‐origin fish tending to run earlier than wild fish. Although the stocking exercise was useful in terms of maximizing freshwater production, concerns over the quality of stocked‐origin recruits and the long term consequences for productivity are highlighted.     

pH preference and avoidance responses of adult brook trout Salvelinus fontinalis and brown trout Salmo trutta

The pH preferred and avoided by wild, adult brook trout Salvelinus fontinalis and brown trout Salmo trutta was examined in a series a laboratory tests using gradual and steep‐gradient flow‐through aquaria. The results were compared with those published for the observed segregation patterns of juvenile S. fontinalis and S. trutta in Pennsylvania streams. The adult S. trutta tested showed a preference for pH 4·0 while adult S. fontinalis did not prefer any pH within the range tested. Salmo trutta are not found in Pennsylvania streams with a base‐flow pH < 5·8 which suggests that S. trutta prefer pH well above 4·0. Adult S. trutta displayed a lack of avoidance at pH below 5·0, as also reported earlier for juveniles. The avoidance pH of wild, adult S. fontinalis (between pH 5·5 and 6·0) and S. trutta (between pH 6·5 and 7·0) did not differ appreciably from earlier study results for the avoidance pH of juvenile S. fontinalis and S. trutta. A comparison of c.i. around these avoidance estimates indicates that avoidance pH is similar among adult S. fontinalis and S. trutta in this study. The limited overlap of c.i. for avoidance pH values for the two species, however, suggests that some S. trutta will display avoidance at a higher pH when S. fontinalis will not. The results of this study indicate that segregation patterns of adult S. fontinalis and S. trutta in Pennsylvania streams could be related to pH and that competition with S. trutta could be mediating the occurrence of S. fontinalis at some pH levels.     

Effects of predation risk on population variation in adult size in a stream-dwelling isopod

I used a combination of laboratory experiments and field surveys to examine the role that population-specific predation risk may play in shaping the life history strategy of a stream-dwelling isopod Lirceus fontinalis. Two focal populations were identified that were exposed to different predator types. The first population was exposed to larvae of the streamside salamander (Ambystoma barbouri) and the second to banded sculpin (Cottus carolinae). A laboratory experiment, in which different size classes of prey were offered simultaneously to individual predators, revealed that L. fontinalis suffered greatest mortality risk at small sizes with A. barbouri. Alternatively, with C. carolinae the risk of mortality was independent of size. Life history theory predicts that L. fontinalis from populations exposed to the gape-limited salamander larvae should be larger at maturity relative to individuals from populations exposed to C. carolinae. Field surveys on the two focal populations both within 1 year and across 4 years supported this prediction. Four other populations, two exposed to streamside salamander larvae and two to fish, provided additional support for the prediction. I concluded that L. fontinalis exhibited an adaptive response in size at maturity in response to population-specific predation risk. I then used gut content assays of the major predators to assess whether the population-specific life history strategies adopted by L. fontinalis were successful in avoiding predation.     

Satiation meal and the effects of meal and body sizes on gastric evacuation rate in brook trout Salvelinus fontinalis fed commercial pellets

Gastric evacuation (GE) experiments were performed on brook trout Salvelinus fontinalis fed commercial food pellets. The experiments included small fish (36 g; 15 cm total length, L_T) fed meals of 0·2, 0·4 and 0·8 g and large fish (152 g; 23 cm) fed meals of 0·8, 2·0 and 4·0 g at temperatures ranging from 15·1 to 18·2° C. The stomach contents were thereafter sampled and weighed at 3 h intervals until the first empty stomach was observed. The course of GE was examined by use of a general power function of the data that revealed that the square‐root function described the GE rate (GER) by the current stomach content mass independently of original meal size. Using the square‐root function, the relationship between GER and fish size was described by a power function of fish length, whereas the effect of temperature was described by a simple exponential function. GER of the commercial pellets fed to S. fontinalis could thus be described by (g h^?1), where S_t is stomach mass (g) at time t (h), L is total fish length (cm) and T is temperature (° C). The result of this study should provide a useful tool for planning of feeding regimes in production of S. fontinalis by optimizing growth and minimizing food waste.     

To fast or feed: an alternative life history for anadromous brook trout Salvelinus fontinalis overwintering within a harbour

The seasonal feeding pattern of sea‐run brook trout Salvelinus fontinalis was studied from November to May 2010–2012 in Antigonish Harbour, Nova Scotia, Canada (45° 38′ N; 61° 55′ W). Sixty‐three S. fontinalis (mean ± s.d . fork length = 330 ± 70 mm and mass = 536 ± 351 g) captured had fed predominantly on fishes (Fundulidae and Gasterosteidae). Percentage of empty stomachs was highest during autumn (18%) and winter (22%) and lowest in spring (7%). Stomach fullness increased from autumn to a maximum during winter, relating to near‐zero body temperatures which may have effectively stopped gastric evacuation. Although feeding occurred during winter (December to March), consumption rates were calculated as negative values, and subsequently returned to positive values in spring (April to May). The over‐winter life‐history strategy of this sea‐run S. fontinalis population appears to be a feeding marine migration in which fish continually increase body condition, representing an alternative to the more common overwintering strategy of starvation in fresh water until spring.     

Effect of feeding under-year zander (Sander lucioperca) on size, body mass and body composition before and after wintering

Zander (Sander lucioperca) is one of the most important fish species in the German inland fishery. As a pelagic predator, zander is able to regulate populations of small cyprinids and perch and is therefore used in ‘top‐down’ projects to directly control the food web. At present, natural waters are mainly stocked with one‐summer‐old zander from pond hatcheries; however, such fish are poor stocking material due to low survival rates, presumably because of their small size. Two groups of 50‐day‐old zander fingerlings reared in a recirculation system (T = 23 °C) were used. For 90 days one group (FG 1) was fed with a commercial dry feed (Trouvit Pro Aqua Brut), and the other group (FG 2) was fed with chironomides (Chironomus spp.). The fat content of the dry feed was increased with fish oil supplemented up to 22%. After the rearing period the FG 1 fishes attained the largest size and fat content. A pond with a surface of 667 m² and a mean depth of 0.8 m was stocked with 24‐tagged zander (12 of each feeding group) for wintering. After the winter period of 176 days, survival rate of each feeding group was 83.3%. The fatty acid composition in the membrane lipids of group 2 changed dramatically. In contrast to previous investigations, presented results indicate that the survival rate is not influenced by fish size (weight and length). The crucial factors for high survival rates of zander during wintering are the crude tissue fat content (>5%) and its fatty acid composition.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

Effects of habitat size and isolation on species immigration–extinction dynamics and community nestedness in a desert river system

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Isolation and cross‐familial amplification of 41 microsatellites for the brook charr (Salvelinus fontinalis)

The brook charr (Salvelinus fontinalis; Osteichthyes: Salmonidae) is a phenotypically diverse fish species inhabiting much of North America. But relatively few genetic diagnostic resources are available for this fish species. We isolated 41 microsatellites from S. fontinalis polymorphic in one or more species of salmonid fish. Thirty‐seven were polymorphic in brook charr, 15 in the congener Arctic charr (Salvelinus alpinus) and 14 in the lake charr (Salvelinus namaycush). Polymorphism was also relatively high in Oncorhynchus, where 21 loci were polymorphic in rainbow trout (Oncorhynchus mykiss) and 16 in cutthroat trout (Oncorhynchus clarkii) but only seven and four microsatellite loci were polymorphic in the more distantly related lake whitefish (Coregonus clupeaformis) and Atlantic salmon (Salmo salar), respectively. One duplicated locus (Sfo228Lav) was polymorphic at both duplicates in S. fontinalis.     

Updated assessment of hatchery‐reared pallid sturgeon (Forbes & Richardson, 1905) survival in the lower Missouri River

As pallid sturgeon, Scaphirhynchus albus (Forbes & Richardson, 1905), natural reproduction and recruitment remains very minimal in the lower Missouri River from Gavins Point Dam (river kilometer [rkm] 1305.2) to the confluence with the Mississippi River (rkm 0.0), hatchery supplementation and river‐wide monitoring efforts have continued. Annual survival estimates of hatchery‐reared pallid sturgeon stocked in the lower Missouri River were previously estimated during 1994–2008. Low recapture rates prior to 2006 limited the data available to estimate survival, which resulted in considerable uncertainty for the estimate of annual survival of age‐1 fish. Therefore, the objective was to provide more precise estimates of annual survival of pallid sturgeon using five additional years of stocking and sampling. The Cormack‐Jolly‐Seber model structure provided in program MARK was used to estimate the age‐specific survival estimates. Over 135 000 hatchery‐reared pallid sturgeon were released during 1994–2011 and recaptured at a rate of 1.9%, whereby estimates for the annual survival of age‐0 (Ø = 0.048) and >age‐1 (Ø = 0.931) were similar to those previously reported, but the age‐1 (Ø = 0.403) survival estimate was 52% lower. Post hoc analysis using time‐specific survival estimates indicated lower survival for age‐1 fish post‐2003 year classes, relative to the pre‐2002 year classes. An analysis confirms that hatchery‐reared pallid sturgeon continue to survive in the wild. However, low survival during the first 2 years of life is a management concern as efforts are aimed at maximizing genetic diversity and population growth. A follow‐up analysis also demonstrated the variability of capture rates and survival over time, which reinforces the need to continue to monitor and evaluate mark‐recapture data. The mark‐recapture efforts have provided demographic parameter estimates that remain a critical component for species recovery as these data are incorporated into population models.     

Trophic structure, species richness and biodiversity in Danish lakes: changes along a phosphorus gradient

1. Using data from 71, mainly shallow (an average mean depth of 3 m), Danish lakes with contrasting total phosphorus concentrations (summer mean 0.02–1.0 mg P L?l), we describe how species richness, biodiversity and trophic structure change along a total phosphorus (TP) gradient divided into five TP classes (class 1–5: <0.05, 0.05–0.1, 0.1–0.2, 0.2–0.4,> 0.4 mg P L?1).
2. With increasing TP, a significant decline was observed in the species richness of zooplankton and submerged macrophytes, while for fish, phytoplankton and floating‐leaved macrophytes, species richness was unimodally related to TP, all peaking at 0.1–0.4 mg P L?1. The Shannon–Wiener and the Hurlbert probability of inter‐specific encounter (PIE) diversity indices showed significant unimodal relationships to TP for zooplankton, phytoplankton and fish. Mean depth also contributed positively to the relationship for rotifers, phytoplankton and fish.
3. At low nutrient concentrations, piscivorous fish (particularly perch, Perca fluviatilis) were abundant and the biomass ratio of piscivores to plankti‐benthivorous cyprinids was high and the density of cyprinids low. Concurrently, the zooplankton was dominated by large‐bodied forms and the biomass ratio of zooplankton to phytoplankton and the calculated grazing pressure on phytoplankton were high. Phytoplankton biomass was low and submerged macrophyte abundance high.
4. With increasing TP, a major shift occurred in trophic structure. Catches of cyprinids in multiple mesh size gill nets increased 10‐fold from class 1 to class 5 and the weight ratio of piscivores to planktivores decreased from 0.6 in class 1 to 0.10–0.15 in classes 3–5. In addition, the mean body weight of dominant cyprinids (roach, Rutilus rutilus, and bream, Abramis brama) decreased two–threefold. Simultaneously, small cladocerans gradually became more important, and among copepods, a shift occurred from calanoid to cyclopoids. Mean body weight of cladocerans decreased from 5.1 μg in class 1 to 1.5 μg in class 5, and the biomass ratio of zooplankton to phytoplankton from 0.46 in class 1 to 0.08–0.15 in classes 3–5. Conversely, phytoplankton biomass and chlorophyll a increased 15‐fold from class 1 to 5 and submerged macrophytes disappeared from most lakes.
5. The suggestion that fish have a significant structuring role in eutrophic lakes is supported by data from three lakes in which major changes in the abundance of planktivorous fish occurred following fish kill or fish manipulation. In these lakes, studied for 8 years, a reduction in planktivores resulted in a major increase in cladoceran mean size and in the biomass ratio of zooplankton to phytoplankton, while chlorophyll a declined substantially. In comparison, no significant changes were observed in 33 ‘control’ lakes studied during the same period.     

Spawning and reproductive patterns of six exploited finfish species from the Arabian Sea, Sultanate of Oman

The spawning periods, sex ratio and length at first maturity were determined for six commercially important finfish species collected from Oman's industrial trawl fishery. During 1 year of monthly sampling, biological data were collected on 5586 fish. ancova was used to compare the length–weight relationships between sexes. There were sex‐specific differences in length‐weight relationships for three of the six species examined. One serranid, Epinephelus diacanthus, and a sparid, Pagellus affinis, had sex ratios significantly different from the expected ratio of 1 : 1. Size–frequency data revealed more male fish in the larger size classes for E. diacanthus, P. affinis and Nemipterus japonicus. Data on the development of female gonads and Gonado‐somatic Index (GSI) revealed that the six species spawned at different times of the year. With the exception of female E. diacanthus, all fish had low GSI values during the onset of the summer monsoon period in June. The proportion of immature vs mature fish in the sample differed among species. For two species, Lethrinus nebulosus and Cheimerius nufar, more than 40% of the fish sampled (both sexes) were immature. Approximately 33% female and 41% male P. affinis were also immature. Management options such as temporal and area closures are discussed in light of the large number of immature fish caught by the fishery.     

Aggression in imported fire ants: an explanation for shifts in their spatial distributions in Southern United States?

Abstract 1. The imported fire ants, Solenopsis invicta Buren (red), S. richteri Forel (black), and their hybrids (S. invicta × S. richteri) are sympatric congeners with overlapping but shifting spatial distributions in northern Alabama and Mississippi, United States. 2. The abilities of workers of the three ant forms (or species) to recognise non‐nestmate conspecifics and heterospecifics were compared in separate aggression bioassays using three group sizes of competitor numbers (battles): 1‐1, 5‐1, and 5‐5 resident–intruder battles. 3. Workers of all three forms showed significant aggression toward non‐nestmate conspecifics and heterospecifics. The results of the intra‐specific aggression bioassays showed that S. invicta had the lowest aggression threshold, whereas S. richteri was the least aggressive. Survival rates 1 day after intra‐specific encounters were higher for S. richteri than for S. invicta or hybrids, consistent with its lower aggressiveness. 4. In inter‐specific interactions, S. invicta workers showed the greatest aggression and were more aggressive towards S. richteri than to hybrids. Furthermore, survival 1 day after inter‐specific encounters was highest for S. invicta workers, whether they were the intruders or residents. The hybrid form was intermediate between both parental forms in aggression and post‐aggression survival. 5. The live 1‐1 and 5‐5 battles produced similar and consistent results, but differences were amplified in the 5‐5 battles, suggesting an effect of group size on aggression. These results may explain the observed shifts in the spatial distributions of the three forms and range restriction of S. richteri in southern United States.     

Temperature‐dependent competition between juvenile salmonids in small streams

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Seasonal resource selection by blue suckers Cycleptus elongatus

Telemetry data were used to quantify seasonal resource selection and rank‐preferred habitat types for blue suckers Cycleptus elongatus in the middle Missouri River and its tributaries, Iowa, Nebraska, South Dakota, U.S.A. Habitat diversity required by C. elongatus to complete its annual life‐history cycle was evident by interseasonal variation in habitat selection. Tributaries and unchannelized portions of the middle Missouri River system were selected during the inferred spawning season (spring), habitats along shorelines in the channelized middle Missouri River were selected for postspawning (summer) and areas of reduced current were generally selected for the period preceding spawning (autumn). This study provides insight into mature C. elongatus behaviour throughout its annual life cycle. Furthermore, these results document the interseasonal variability in habitat selection of C. elongatus and highlight the need for development of habitat selection models for other imperilled fish species in aquatic systems affected by anthropogenic developments.     

Seasonal temperature and precipitation regulate brook trout young‐of‐the‐year abundance and population dynamics

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