Small Sunfish Research Articles

Ontogenetic shift in foraging habit of ocean sunfish Mola mola from dietary and behavioral studies

The ocean sunfish (Mola mola) is typically considered to feed on gelatinous zooplankton, but reports in the literature describe various benthic organisms being found in their stomachs. This might reflect ontogenetic dietary shift, as little was known about the foraging habit of this species. We examined their foraging habits using dietary analyses in combination with a behavioral study in Iwate, Japan (39°22′N, 141°58′E) from 2009 to 2010. Our stomach content analyses (n = 17, 31–250 cm total length) suggested that small sunfish ( 200 cm) feed on jellyfish. Larger sunfish showed higher values of both carbon and nitrogen stable isotope ratios. Deployment of accelerometers and animal-borne cameras on small sunfish in July (49–58 cm, n = 5) suggested their possibility of feeding, while they stayed near the seabed. This indicates that small sunfish might feed on benthic preys. Deployment of accelero-magnetometers on large sunfish in July (84–164 cm, n = 4) clarified that the large sunfish in July swam back and forth between the surface and deep water (>100 m). Temporary decelerations, which were considered to be associated with feeding of planktonic prey, were observed in deep water. Whereas deployment of accelero-magnetometers on large sunfish in November (105 cm, n = 3) showed several bursts, they swam within the mixed layer (0–100 m), which might be associated with chasing of rapid prey. These results suggest that ocean sunfish have heterogeneous diets depending on their body size and possibly season.

Injury and Mortality Induced by Four Hook Types on Bluegill and Pumpkinseed

Tackle manufacturers have responded to concerns regarding hooking injury and mortality by attempting to design and market hooks that are less damaging to fish (e.g., circle hooks). To date, studies investigating circle hooks have been primarily restricted to large marine species. We compared the injury and short-term (72-h) mortality of bluegills Lepomis macrochirus and pumpkinseeds L. gibbosus angled using number-6 circle hooks and three other conventional hook types (aberdeen, wide-gap, and baitholder) across three water temperatures (18, 22, and 26°C). Unlike other hook types, circle hooks were never lodged in the gullet, but they were frequently lodged in the eye. Some fish captured on conventional hooks were hooked deeply in the gullet, necessitating line cutting for release. Incidences of bleeding were low using all hook types, and when not lodged in the gullet, all hooks were generally easy to remove. Anatomical hooking locations differed among small (<145-mm) and large (>145-mm) bluegills for all hook types but not among pumpkinseeds. Hooking depth differed between small and large fish of both species captured on circle hooks; smaller fish were hooked more deeply. Mortality in both species was negligible at all water temperatures except for bluegills at 26°C (3% mortality). Bluegills that died were smaller than those that survived. Our results confirm the supposition that circle hooks are less susceptible to deeply hooking fish in the gullet. However, circle hooks permanently impaired vision of up to 22% of the fish, much more than for other hooks types. Although efficient at minimizing injury and mortality in marine fish, our study suggests that circle hooks perform similarly to more conventional hook types in fisheries for small sunfish.

Lead concentrations in fish and river sediments in the old lead belt of Missouri.

Although active mining activities in the Old Lead Belt of Missouri ceased in 1968, old tailings piles remain and continue to impact the lead (Pb) concentrations in sediments and fish in the Big River and Flat River Creek in Missouri. A 3-year study was conducted that examined the Pb concentrations in organic-rich sediments, fish fillets, and fish whole bodies at 13 sites in these two rivers. The results showed that the sediments had significantly elevated Pb concentrations near and beyond the tailings piles compared with control sites upstream from the mining activity. The results also showed the Pb concentration in sediments correlated well with Pb concentrations in suckers (r = 0.86, p < 0.0001) and small sunfish (r = 0.82, p < 0.0001). Annual average Pb concentrations in sucker fillets ranged up to 0.67 microg/g (wet weight basis) and exceeded the World Health Organization guidelines of 0.3 microg/g (wetweight basis) at many of the sample sites. Fillets of bass, which feed at a higher trophic level, had much lower Pb concentrations than either suckers or small sunfish, and no bass fillet samples exceeded the WHO guideline. Whole body Pb concentrations in small sunfish were also determined and correlated strongly (r = 0.92, p = 0.0004) with sediment concentrations before leveling at a maximum of approximately 20 microg/g (wet weight basis).

Spatial and temporal variation in fish assemblages of drying stream pools: The role of abiotic and biotic factors

Droughts and summer drying create unusual temporary aquatic habitats in the form of isolated pools in many small streams around the world. To examine spatial and temporal variation in fish community structure of drying stream pools, their relation to abiotic environmental variables, and associations among species, fish were sampled during summer 1995 and 1996 from pools of four streams in the Ozark mountains, Arkansas, USA. Redundancy analysis of physical-chemical variables showed significant differences among stream sites, but no significant difference between years or stream site by year interaction. Stream sites separated consistently along axes one (habitat heterogeneity) and two (temperature/canopy cover) in both years. Redundancy analysis of fish species-size class densities showed a significant stream site by year interaction. Groupings of stream sites based on fish assemblages were not well explained by physical-chemical variables measured at the pool scale, but were related to location within the drainage basin, and these groupings differed between years. There were 27 (15.8%) and 10 (5.8%) significant associations found among fish species-size classes in 1995 and 1996, respectively, and all but two significant associations in 1995 were positive. Pool depth, habitat heterogeneity, pool size and dissolved oxygen/canopy cover were important local abiotic factors depending on response variables examined. In both years, large fish total density, large central stoneroller density (≥80 mm TL), and small sunfish (<80 mm TL) density were positively related to pool depth. Otherwise, there was no consistent relationship between physical-chemical variables and dependent variables (fish density and species richness) within a year or between years for a given dependent variable. These results support the hypothesis that local abiotic factors are important in structuring fish assemblages in harsh environments, but the importance of those factors varies temporally, and regional influences appear to override local abiotic conditions as factors structuring fish assemblages in drying stream pools. Predation by terrestrial vertebrates may also be an important factor structuring these fish assemblages that has been largely overlooked.

Link Strength in Lake Littoral Food Webs: Net Effects of Small Sunfish and Larval Dragonflies

To show how predaceous fish and dragonflies affect benthic community structure, we conducted enclosure experiments in the littoral zone of Bays Mountain Lake, Tennessee. A "natural" benthic assemblage was subjected to all combinations of 2 densities of 3 predator treatments--small sunfish, 0 or 4/m²; large dragonfly larvae, 0 or 15/m²; and dragonfly eggs at 2 densities, 90 or 900/m². Treatments were assigned randomly in each of 6 spatiotemporal blocks. Net effects of predation over 4 mo show that small sunfish had "strong" effects (>50% reduction of densities) on triclads, large daphnid cladocerans, and snails, and "moderate" effects (50% > reduction > 25%) on small dragonflies and ostracods; all these effects were statistically significant, except the one on ostracods. Large dragonflies had moderate non-significant effects on triclads and ostracods. There were no significant increases in prey density associated with fish predation; but chydorid cladocerans and midge larvae showed "weak" non-significant increases that might be caused by "indirect effects" of fish predation on invertebrate predators or grazers. There were only 2 significant interaction terms indicative of "higher-order interactions": Fish × Dragonfly on the large daphnid Simocephalus, and Fish × Egg Density on snails, which were associated with relatively low probabilities. In both cases, the net effect of dragonflies was to reduce prey densities more when fish were present.

Predation, Density Dependence, and Life Histories of Dragonflies: A Field Experiment in a Freshwater Community

To characterize interactions among fish and dragonfly predators that influence population dynamics and community structure, we conducted enclosure/exclosure experiments (April-October) using 1.8-m diameter cylinders at 1-m depth in the littoral zone of Bays Mountain Lake, Tennessee. A nearly natural invertebrate assemblage colonized allochthonous detritus through 1.8-mm mesh screen from April to May when further colonization was restricted by 0.5-mm mesh. Treatments introduced in mid May consisted of all combinations of two densities of two predators--small sunfish (Lepomis macrochirus), 0 or 4 individuals/m 2 ; senior-year-class dragonfly larvae (Epitheca cynosura), 0 or 15 individuals/m 2 --acting on cohorts of junior-year-class E. cynosura hatching from eggs stocked at two densities, 90 or 900/m 2 . Treatments were assigned randomly in each of six spatiotemporal blocks: two in 1987 and four in 1988. Junior-year-class Epitheca hatching from eggs stocked at different densities ...

Morphological constraints in the piscivore‐planktivore interaction: Implications for the trophic cascade hypothesis

Results are presented from a 3‐yr, replicated pond study examining how differential planktivore vulnerability can influence planktivore responses to piscivory and ultimately plankton responses via cascading effects. Without piscivores, planktivory from a multispecies planktivore assemblage (fathead minnow Pimephales promelas and pumpkinseed sunfish Lepomis gibbosa) produced the well‐documented plankton responses. Chaoborus densities, zooplankton biomass, and cladoceran and copepod mean body sizes were reduced, and chlorophyll concentrations and gross primary production were enhanced. With piscivores, levels of planktivory were depressed due to elimination of the highly vulnerable minnow and a shift in the size structure of the relatively less vulnerable sunfish toward larger, benthivorous sizes. Hence, zooplankton biomass and cladoceran and copepod mean body sizes were increased, and chlorophyll concentrations and gross primary production were reduced. However, juvenile sunfish remained abundant in ponds containing piscivores, so Chaoborus densities remained low and small cladocerans (Bosmina and Ceriodaphnia) were abundant. Moreover, Daphnia biomass, mean body size, and number of days present during the final year in these ponds were negatively correlated with small sunfish biomass, indicating that piscivory did not simply cancel out the effects of planktivory.

Growth‐Related Constraints on Diet Selection by Sunfish

Diet selectivity by the planktivorous bluegill sunfish (Lepomis macrochirus) is correlated with the fishes' visual resolution. Poor visual capabilities are thought to impose a constraint on the ability of small sunfish to chose prey. Li et al. (1985) suggested that sunfish subjected to conditions of poor visual resolution select prey in accordance with a model in which prey choice is determined by apparent prey size. Further, enhanced visual resolution permitted large sunfish to assess absolute prey size more accurately than could smaller conspecifics and, consequently, large—sized sunfish consumed relatively more large prey than was predicted by the apparent size model. Here, we examine the visual capabilities and prey selection of very small sunfish, extend the analyses of Li et al. by determining the optimal diet sets for the fish used in their study, and compare prey selection by all sunfish size categories to that predicted by the apparent size and optimal foraging models. Contrary to the conclusions of Li et al., small sunfish (&lt;20 mm standard length [SL]) were more selective than was predicted by the apparent size model and preferentially attacked small—sized Daphnia. The optimal foraging "prey" model predicted correctly the size—related changes in mean size of prey attacked by sunfish, whereas the apparent size model predicted a preference for large Daphnia for all sizes of fish. Yet, the observed diet breadths were always larger than those predicted by the optimal foraging model. The visual system any energy considerations constrain prey selection in small sunfish but operate in opposite fashions.

Indirect effects of fish on macrophytes in Bays Mountain Lake: evidence for a littoral trophic cascade.

We began this experiment to test specific hypotheses regarding direct and indirect effects of fish predation on the littoral macroinvertebrate community of Bays Mountain Lake, Tennessee. We used 24 m2 enclosures in which we manipulated the presence and absence of large redear sunfish (Lepomis microlophus>150 mm SL), and small sunfish (L. macrochirus and L. microlophus <50 mm SL) over a 16-mo period. Here we report on effects of fish predation on gastropod grazers that appear to cascade to periphyton and macrophytes.Both large redear sunfish and small sunfish maintained low snail biomass, but snails in fish-free controls increased significantly during the first 2-mo of the experiment. By late summer of the first year of the experiment, the difference in biomass between enclosures with and without fish had increased dramatically (>10×). Midway through the second summer of the experiment, we noted apparent differences in the abundance of periphyton between enclosures containing fish and those that did not. We also noted differences in the macrophyte distribution among enclosures. To document these responses, we estimated periphyton cover, biovolume and cell size frequencies as well as macrophyte distributions among enclosures at the end of the experiment. When fish were absent, periphyton percent cover was significantly reduced compared to when fish were present. Periphyton cell-size distributions in enclosures without fish were skewed toward small cells (only 12% were greater than 200 m3), which is consistent with intense snail grazing. The macrophyte Najas flexilis had more than 60 x higher biomass in the fish-free enclosures than in enclosures containing fish; Potamogeton diversifolius was found only in fish-free enclosures. These results suggest a chain of strong interactions (i.e. from fish to snails to periphyton to macrophytes) that may be important in lake littoral systems. This contrasts sharply with earlier predictions based on cascading trophic interactions that propose that fish predation on snails would enhance macrophyte biomass.

The ecological role of chemical stimuli for the zooplankton: predator-induced morphology in Daphnia.

Numerous adaptive predator-induced responses occurred when eight clones representing seven Daphnia (Crustacea: Cladocera) species were tested against three common predators: fourth instar larval phantom midge Chaoborus americanus, adult backswimmer Notonecta undulata, and small sunfish Lepomis macrochirus. The predators were confined within small mesh bags, suggesting that the signal for induction is chemical. The induced responses included longer tail spines, longer heads, smaller bodies, increased egg clutches, and decreased lipid reserves. Each Daphnia species responded to each of the three predators in a unique manner. Induced responses in the above characters showed no significant association. The induced morphological changes are generally consistent with current theories of what is an adaptive response for the various sizes of Daphnia exposed to tactile and visual predators. The abundance of induced responses in these experiments suggests that predator-induced responses are a widespread and ecologically important phenomenon of the freshwater zooplankton.

Effects of Seeding Reservoir Fluctuation Zones on Young-of-the-Year Black Bass and Associated Species

Abstract Fertilized rye grew well in the fluctuation zone of Lake Nottely, a tributary reservoir in northern Georgia, during fall and winter drawdown providing cover at a materials cost of $38/hectare. Considering the three species of black bass (Micropterus salmoides, Micropterus punctulatus, and Micropterus dolomleui) in Lake Nottely together, young-of-the-year (y-o-y) bass were consistently more abundant in the flooded, seeded areas than in the unseeded control areas from the middle of June to the middle of July; however, in late summer the number of y-o-y black bass in both areas was similar. Seeded areas had a slight but positive effect on growth of the bass in that they were 8% larger and 13% heavier than y-o-y bass in the control area by the end of the summer. Numbers of aquatic insects and small sunfish were more abundant in the seeded areas. Y-o-y black bass were more piscivorous in the seeded areas, and there was no apparent negative effect on y-o-y bass due to sunfish (Lepomis spp.), y-o-y whit...