Gape Height Research Articles

The Role of Size in Trophic Niche Separation between Two Groundfish Predators in Alaskan Waters

AbstractTo minimize negative effects of competition, predators must divide resources along one or more niche dimensions. Trophic niche separation is a principal method of resource partitioning in marine environments and is often assessed by quantifying diets of fishes with similar lengths. Taxonomic differences in allometric growth, however, suggest that the degree of gape limitation at a given body size varies among species. We studied resource overlap between two potential competitors in Southeast Alaska: Pacific Halibut Hippoglossus stenolepis and Arrowtooth Flounder Atheresthes stomias. We quantified length–gape relationships and assessed effects of fork length (FL; cm), gape height (GH; mm), and gape width (GW; mm) on interpretations of trophic niche separation. Gape sizes for Arrowtooth Flounder were nearly twice those of Pacific Halibut at similar FLs. Predators with smaller gapes exhibited moderate to high dietary overlap ( = 0.554; = 0.731), whereas those with larger gapes had lower dietary overlap and distinct diet compositions ( = 0.421; = 0.265). These findings suggest that the greatest potential for food competition is among small‐bodied Arrowtooth Flounder and relatively large‐bodied Pacific Halibut, with trophic niche separation increasing as these predators progress throughout their ontogeny. When comparing resource use among predators with similar FLs, we found high dietary overlap but significantly different diet compositions ( = 0.657). Thus, we postulate that GW is more appropriate than FL when quantifying trophic niche separation among sympatric fishes, especially when disparate length–gape relationships are detected. Studies of resource partitioning would benefit from examining multiple size metrics and explicitly considering differences in allometric growth, thereby addressing potential mechanisms for resource overlap among hypothesized competitors.

Is Bigger Better? Evaluation of Size‐Selective Predation on Age‐0 Walleye

AbstractThe success of fish stocking programs is dependent on poststocking survival. However, survival can be influenced by size‐selective mortality, where larger individuals are expected to be less vulnerable to predation. Walleye Sander vitreus is a commonly stocked sport fish, but little is known regarding the role of size‐selective predation on age‐0 Walleye (90–290 mm TL) that are stocked in the presence of various piscivores with different foraging strategies and morphological features. Our objective was to evaluate whether the TL of consumed age‐0 Walleye was related to predator TL, predator gape height, or the probability of predation. We also assessed whether length distributions varied among stocked, recaptured, and consumed age‐0 Walleye. Data for the food habits of Largemouth Bass Micropterus salmoides, Smallmouth Bass M. dolomieu, Northern Pike Esox lucius, adult Walleye, and Muskellunge Esox masquinongy were collected from East Okoboji Lake and West Okoboji Lake, Iowa, USA during fall 2015–2017. Over the course of the study, 301 age‐0 Walleye were recovered from 3,514 predator stomachs. Mean, maximum (85th percentile), and minimum (15th percentile) TL of the consumed age‐0 Walleye were not related to the TL or gape height of the predators (P > 0.05), but the probability of predation decreased by 0.02 for every 10‐mm increase in the TL of the age‐0 Walleye. The length distributions indicated that consumed Walleye were generally smaller, whereas recaptured Walleye tended to be larger than stocked fish were. Our approach to evaluating poststocking size‐selective predation on age‐0 Walleye furthers the current understanding of the importance of size at stocking relative to stocking success and suggests that stocking Walleye > 220 mm is economically advantageous for reducing predation risk.

Biometric relationships between age-0 walleye Sander vitreus total length and external morphometric features

In fishes, predation often is size-specific, with decreasing predation risks associated with larger body size. However, consumed fishes observed during diet analysis are often partially digested with only a variety of different body parts identified, making it challenging to determine original body size at consumption and size-specific predation risk. Therefore, the objective of this study was to assess linear relationships between total length and 10 body components of age-0 walleye Sander vitreus. Ten morphometric features (spinal column: first anterior vertebrae before the caudal bone to the first vertebrae posterior of the cranium (SC), length between anterior edge of the rostrum and posterior margin of operculum margin (RO), length between anterior edge of the rostrum to the origin of the dorsal fin (RD), gape height (GH), length of the base of the anal fin (AF), height of the caudal fin (CF), orbit height (OH), orbit width (OW), base of anterior (AD), and posterior (PD) dorsal fins) were measured from 61 age-0 walleye ranging from 119 to 275 mm total length. Walleye total length was positively related to all ten morphometric features: spinal length (R2 = 0.90), gape height (R2 = 0.84), orbit width (R2 = 0.52), and orbit height (R2 = 0.56), base length of anterior dorsal fin (R2 = 0.85), base of posterior dorsal fin (R2 = 0.80), base of anal fin (R2 = 0.73), length between rostrum and first dorsal fin (R2 = 0.91), caudal height (R2 = 0.46), and rostrum to posterior margin of the operculum (R2 = 0.95). Most age-0 walleye (87%) consumed by predators had at least one morphometric feature that was measurable, with 42% of the measurable features having coefficient of determination value of at least 0.80 with total length. Length-frequency histograms developed from total lengths estimated from morphometric features were all similar to direct total length measurements, indicating that they provided a reliable metric of size distribution. Establishing biometric relationships for morphometric features that can be directly measured provide an alternative to back-calculation of prey size and provide information that aids in understanding complex relationships between recruitment, predation, and prey size.

Quantifying trophic interactions and niche sizes of juvenile fishes in an invaded riverine cyprinid fish community

AbstractQuantifying feeding interactions between nonindigenous and indigenous fishes in invaded fish communities is important for determining how introduced species integrate into native food webs. Here, the trophic interactions of invasive 0+ European barbel Barbus barbus (L.) and the three other principal 0+ fishes in the community, Squalius cephalus (L.), Leuciscus leuciscus (L.) and Phoxinus phoxinus (L.), were investigated in the River Teme, a River Severn tributary in Western England. B. barbus has been present in the River Teme for approximately 40 years. Analyses of stomach contents from samples collected from three sites between June and September 2015 revealed that, overall, fishes displayed a generalist feeding strategy, with most prey having low frequency of selection. Relationships of diet composition versus body length and gape height were species‐specific, with increasing dietary specialisms apparent as the 0+ fishes increased in length and gape height. The trophic niche size of invasive B. barbus was always significantly smaller than S. cephalus and L. leuciscus and was significantly smaller than P. phoxinus at two sites. This was primarily due to differences in the functional morphology of the fishes; 0+ B. barbus were generally restricted to foraging on the benthos, whereas the other fishes were able to forage on prey present throughout the water column. Nevertheless, the invasive B. barbus were exploiting very similar prey items to populations in their native range, suggesting these invaders were strongly pre‐adapted to the River Teme and this arguably facilitated their establishment and invasion.

Effects of supplemental wild zooplankton on prey preference, mouth gape, osteological development and survival in first feeding cultured larval yellow tang (Zebrasoma flavescens)

Yellow Tang, Zebrasoma flavescens, are a popular species in the aquarium industry. Currently, all Yellow Tang are collected from wild stocks, with collection in Hawaii accounting for the bulk of the individuals found in the trade. Although culture of this species has recently been successful, survival remains low and mass mortality events, particularly associated with the transition from endogenous to exogenous feeding, continue to be observed. Improving survival and gaining a more thorough understanding of the drivers behind early prey selection, such as mouth gape and osteological development, are of primary importance in advancing the culture of Yellow Tang. For the present study, a supplemental size-sorted mixture of wild zooplankton was offered in conjunction with and compared to the cultured nauplii of Parvocalanus crassirostris to determine the effect on survival. Prey selection was determined via metabarcoding. Osteological development was documented and mouth gape measured from 1 to 6 days post hatch to better determine appropriate prey types and size ranges. Survival was significantly lower in larvae fed cultured nauplii supplemented with wild zooplankton, 6.9% v. 18.5%. Regardless of the presence of wild zooplankton, larvae preferentially selected for P. crassirostris based on molecular determination of prey consumption. However, it is likely that protozoans were mostly excluded from this molecular analysis possibly due to the type of primer selected. Mouth gape height increased by approximately 100 μm and mouth gape width by approximately 55 μm, during the study period. Although no significant difference in growth was calculated between treatments, larvae receiving the wild zooplankton supplement diet were observed to grow at a slightly slower rate than larvae being fed only cultured copepod nauplii. The rate at which the mouth gape increased suggests that the currently cultured and utilized copepod nauplii may not be an optimally sized prey item shortly after first feed. These findings indicate that first feeding Yellow Tang larvae are capable of consuming larger prey at an earlier age than they have been offered in previous rearing attempts, but that the larvae may continue to benefit from the presence of less elusive prey items, such as ciliates or rotifers, due to their delayed osteological development. Osteological development was slow with very little ossification occurring by 6 days post hatch and only in those larvae fed solely cultured nauplii. However, their selection of copepod nauplii suggests that some aspect of this prey item is preferential to the first feeding larvae and may elicit a hunting response essential to initiating exogenous feeding.

Trophic dynamics of few selected nearshore coastal finfishes with emphasis on prawns as prey item

A trophic dynamic study of marine finfishes was undertaken based on stomach content analysis of twenty four species (N = 1742) collected from the nearshore coastal waters off Goa, west coast of India (15°29′07.6″ N to 15°34′44.3″ N, and 73°38′10.5″ E to 73°46′03.1″ E) during November 2010 to May 2013. This study aimed to thoroughly understand the feeding attributes of finfishes, and comprehend the possible effects of bycatch-related loss of biomass on trophic ecology. The study assessed diet preferences of the finfishes, their feeding guilds, significance of prawns as prey items, and the influence of mouth parts in prey selection. Altogether 84 prey taxa were identified from the stomach contents. Percentage Index of Relative Importance (IRI) values revealed that zooplankton (34.74), prawns (21.71), phytoplankton (19.80), and teleosts (18.62) were the major prey categories, and, among prawns, Metapenaeus dobsoni (%IRI = 19.34) was the single-most important prey item. Cluster analysis revealed three major trophic guilds namely ‘teleost feeders’ (mean Trophic Level (TrL) = 4.06 ± 0.42; mean B = 0.46 ± 0.24), ‘zooplankton feeders’ (mean TrL = 3.43 ± 0.29; mean B = 0.23 ± 0.13), and ‘prawn feeders' (mean TrL = 3.86 ± 0.25; mean B = 0.48 ± 0.32), with low diet overlap among them. Principal Component Analysis of prey categories and mouth parts of finfishes suggested that zooplanktivory is associated with gill raker density as well as number of gill arches bearing rakers, whereas gape height determined the size of large-sized prey (fish and invertebrates). The study identified M. dobsoni, mysis and teleosts as highly influential prey for predatory finfishes. The present results could be useful to resolve broader issues in fisheries management.

Does morphology predict trophic niche differentiation? Relationship between feeding habits and body shape in four co-occurring juvenile species (Pisces: Perciformes, Sparidae)

Feeding habits, diet overlap and morphological correlates of four juvenile species of the genus Diplodus were investigated during their settlement periods, along the Tyrrhenian coast. Stomach content analysis showed that the diets of D. sargus and D. puntazzo mainly comprised benthic prey such as harpacticoid copepods, amphipods and polychaetes. On the other hand, D. vulgaris and D. annularis fed mainly on planktonic prey such as ciclopoids, calanoids copepods and fish larvae. A biologically significant diet overlap, calculated using the Schoener index, was recorded between D. sargus and D. puntazzo and between D. vulgaris and D. annularis. Morphological characters related to feeding such as gape height and gut length with their relative growth patterns suggested that different trophic preferences have led to a morphological diversification of feeding structures. Therefore, a geometric morphometric outline method, namely Elliptic Fourier Analysis (EFA) was used to examine shape modification of the head and body regions. The multivariate analyses performed on shape descriptors demonstrated that the four species were morphologically distinct due to different feeding habits: the two species which feed mainly on benthic prey presented a discoidal shape, with broad profiles and rounded head; by contrast, the other two species which relied mostly on planktonic prey, presented a streamlined and more elongated body shape.

Body Size Predicts Echolocation Call Peak Frequency Better than Gape Height in Vespertilionid Bats

In most vocalizing vertebrates, lighter animals tend to produce acoustic signals of higher frequency than heavier animals. Two hypotheses propose to explain this negative relationship in vespertilionid bats: (i) mass-signal frequency allometry and (ii) emitter-limited (maximum gape) signal directionality. The first hypothesis, that lighter bats with smaller larynges are constrained to calls with higher frequencies, is supported at the species level. The second hypothesis proposes that in open space, small bats use higher frequencies to achieve narrow sonar beams, as beam directionality increases with both emitter size (maximum gape) and signal frequency. This hypothesis is supported within a comparative context but remains untested beyond a few species. We analyzed gape, body mass, and echolocation data under a phylogenetic comparative framework to test these hypotheses, and considered forearm length as both a proxy for wing design and an alternative measure of bat size. Controlling for mass, we found no support for the directionality hypothesis. Body mass and relative forearm length were negatively related to open space echolocation call peak frequency, reflecting species-specific size differences, but also the influence of wing design and preferred foraging habitat on size-independent species-specific differences in echolocation call design.

Feeding habits and predator-prey size relationships of mandarin fishSiniperca chuatsi(Basilewsky) in a shallow lake, central China

The diet and predator-prey size relationships of mandarin fish Siniperca chuatsi (Basilewsky) in Lake Xiaosihai along the middle reach of the Yangtze River were studied through stomach content analysis. A total of 401 specimens (91539 mm total length) were collected in 2007. The diet was dominated by topmouth gudgeon Pseudorasbora parva, sharpbelly Hemiculter leucisculus, redfin culter Cultrichthys erythropterus, and crucian carp Carassius auratus, with significant seasonal and ontogenetic differences. Ontogenetic variation in diet was apparent that larger prey items such as crucian carp and redfin culter became more common, while smaller prey such as topmouth gudgeon, bitterlings and shrimps gradually declined in the larger fish. Mandarin fish total length (TL) was strongly related to mouth gape width (GW) and gape height (GH). Mandarin fish TL and prey fish TL as well as mandarin fish GW and prey fish body depth (BD) were positively and linearly related for sharpbelly, redfin culter and crucian carp. Strong size selectivity for topmouth gudgeon, sharpbelly and redfin culter as prey indicated that the piscivorous mandarin fish can have potential impact on the population size structure of the three prey fish.

Breeding of the forktail blenny Meiacanthus atrodorsalis: Broodstock management and larval rearing

The present study describes the reproductive behavior of the forktail blenny, Meiacanthus atrodorsalis, and investigates the survival and growth of larvae under various feeding regimes. Two harems of six M. atrodorsalis began spawning about 3 months after acquisition, with egg clutches discovered regularly throughout the study period. Observations of courting behavior suggested that the female initiated courtship with spawning commencing after a series of male displays and courtship encounters. Adult M. atrodorsalis displayed a preference for shelters of single entrance 50-mm PVC pipe, with a 25-mm reduced entrance, for egg laying, while the male took full responsibility for egg care. Larvae measuring 3.11 ± 0.15 mm standard length (SL) and 0.63 ± 0.03 mm body depth (BD) hatched approximately 181 h post fertilization at 28 °C, with a mouth gape height and width of 307.3 ± 11.0 and 263.8 ± 5.5 μm, respectively. Using a feeding protocol consisting of firstly feeding rotifers, and then switching to Artemia nauplii with enriched Artemia meta-nauplii added later on, larvae were observed to reach settlement approximately 35 days post hatching (DPH), measuring 13.54 ± 0.38 mm SL and 3.91 ± 0.27 mm BD. The first feeding trial tested the effect of rotifer density on survival and growth of newly hatched larvae. The results showed no significant difference in either larval survival or growth when fed rotifers at 2, 5, 10 and 20 rotifers mL −1. The second trial investigated the appropriate time to shift from feeding larvae with rotifers (10 mL −1) to Artemia nauplii (3 mL −1). These feeding trials suggest that to optimize survival and growth, newly hatched larvae of M. atrodorsalis should be fed rotifers at a density of between 2 and 20 rotifers mL −1, and that the majority of larvae appeared to display the ability to ingest Artemia nauplii between 6 to 10 DPH.

Why is cannibalism so rare among cultured larvae and juveniles of Pangasius djambal? Morphological, behavioural and energetic answers

The ontogenetic trajectory of Pangasius djambal resembles that of Pangasianodon hypophthalmus. At the start of exogenous feeding (48 h after hatching), larvae of P. djambal (8.5 mm TL, 4.5 mg WM) exhibit a large gape (17.5% TL), they possess long oral spines (100 µm) but no pectoral fins. However, the spines do not overhang from the mouth, and gape height never exceeds body depth, contrary to the situation in P. hypophthalmus. These differences account for why encounters between larvae of P. djambal never lead to the deadly clashes observed in P. hypophthalmus. The rarity of cannibalism in older larvae and juveniles of P. djambal originates from the combination of morphological, behavioural and energetic factors (studied during predation experiments with cannibals from 20 to 70 mm TL): (1) the negative allometry of mouth parts and positive allometry of body depth restrict the logistics of cannibalism from 84.9% TL at 10 mm TL to 56.5% TL at 40 mm TL; (2) cannibals of increasing size prefer prey that are increasingly smaller relative to the logistics of cannibalism (pref:max TL ratios of 99 and 48.5% at 10 and 40 mm TL, respectively); and (3) cannibals need high maintenance food rations (14.9% DM at 6–10 mg DM), they exhibit a rather low gross conversion efficiency (0.36 DM:DM at 6 mg DM), and gain no growth advantage over siblings fed brine shrimp nauplii ad libitum. No single factor suffices to account for the rarity of cannibalism in P. djambal, but their combination almost makes it impossible, even among fish fed submaximally.

Morphological factors behind the early mortality of cultured larvae of the Asian catfish, Pangasianodon hypophthalmus

Cultured larvae of P. hypophthalmus often undergo high mortality that is generally attributed to intense but temporary cannibalism. Here, evidence is provided that mortality is largely independent from feeding or aggressiveness, but is a consequence of the developmental pattern of this species. Embryos grow oral teeth but also long (100 μm) sharp oral spines, which overhang from the mouth and prevent its closure at the start of exogenous feeding (60 haf, hours after fertilization). At the start of exogenous feeding, gape height exceeds body depth (18 versus 14% of the fish total length), so larvae can grasp siblings on any body location. The manoeuvrability and possibility of moving backwards are restricted in the absence of pectoral fins, which develop exceptionally late in this species. Hence, encounters between larvae can be deadly, irrespectively of the propensity of fish to feed or show aggressive behaviours, as was verified during experiments with anaesthetised specimens. The temporary nature of the phenomenon is accounted for by the negative allometric growth of jaw length, the positive allometric growth of body depth, and to the fact that oral spines stop growing after the yolk is fully absorbed (95 haf). Altogether, these traits largely account for why deadly clashes occur among young larvae of P. hypophthalmus, but are restricted to a few hours (clashes) or days (wounds). Implications for improving the larviculture of this species are discussed.

The food and feeding relationships of larval and 0+ year juvenile fishes in lowland rivers and connected waterbodies. II. Prey selection and the influence of gape

The relative importance of taxa‐ and size‐specific prey selection, and the influence of gape on the prey consumed by the larvae and 0+ year juveniles of four fish species were investigated in ‘main river’, ‘marina’ and ‘pond’ macrohabitats in the lower River Trent, England. A general sequence of ontogenetic shifts in food consumption was reflected in the electivity indices of particular prey taxa, partly due to the restrictions imposed by the gape of 0+ year fishes. Certain taxa, however, were consistently selected over others, irrespective of size, suggesting that taxa‐specific, as well as size‐specific, prey characteristics may be important in the selection process. There were significant, positive relationships between maximum prey (zooplankton) length and maximum gape height for larvae, but not for 0+ year juveniles. The majority of fishes, however, consumed prey substantially smaller than the maximum theoretically possible inferred from their gape. The greater size ranges of zooplankton in connected waterbodies compared with main river channels provide suitable prey for a range of developmental steps and fish species, and may, thus, enhance recruitment success.

Size‐dependent predator–prey relationships between perch and their fish prey

Size‐dependent interactions between piscivorous perch Perca fluviatilis (age ≥1 year) and their fish prey age 0 year perch, pikeperch Sander lucioperca and roach Rutilus rutilus in the biomanipulated Bautzen Reservoir indicated that the highest ratio of prey total length (LT) to predator LT was 59%. Perch LT and prey fish LT were positively and linearly related. Perch LT was strongly related with both gape width and gape height. Within the range 80–110 mm LT, the gape height of perch exceeded gape width, while beginning at 120 mm LT the gape width exceeded gape height. The minimum, maximum and mean prey LT and prey body depths of all three prey species increased with increasing predator size, but the increases in mean sizes of perch and pikeperch as prey were less than that of roach. The low limit of the ‘predation window’ observed in this study coupled with results of previous studies on perch in the Bautzen Reservoir indicated that perch had a major impact on the population dynamics of both perch and pikeperch.

Intraspecific variation in Gape–prey size Relationships and Feeding Success During Early Ontogeny in Red Drum, Sciaenops Ocellatus

The relationship between predator gape and prey consumption in laboratory-reared larva and field-caught early juvenile red drum, Sciaenops ocellatus, was investigated in light of the hypothesis that feeding success varies throughout the early life history intervals of marine fishes. We expected the feeding ability of red drum to be more strongly constrained by mouth gape in smaller fish and expected this ability to improve with gape size. To test this hypothesis, field-caught, early juvenile red drum were examined to determine the relationship between gape size and prey size consumed. In field-caught early juveniles, gape (height and width) and prey size consumed (length and width) increased linearly with standard length (SL); however, mean width of prey consumed was only 20–47% of gape width. Furthermore, when regressed on SL, gape width yielded a higher slope than prey width. To further test this hypothesis on less developed, pre-metamorphic fish, age-specific differences in gape, number of prey and size of prey consumed prior to metamorphosis were determined from laboratory-reared red drum larvae. Similar patterns were observed for gape height– and gape width–SL relationships in laboratory-reared red drum larvae. Size of consumed prey increased from three days from hatching (dfh) to 18 dfh. The percentage of feeding larvae also increased from ∼3% at 3 dfh to 97% at 18 dfh. In both field-caught, early juvenile red drum and laboratory-reared larvae, there was little evidence that the size of prey consumed was constrained by mouth gape. It is hypothesized that besides gape size, the development of other features of the feeding mechanism (e.g., hyoid and opercular series) influences prey-capture performance prior to settlement in marine fishes.

Variation in Prey-resource Utilization and Oral Jaw Gape Between two Populations of Largemouth Bass, Micropterus Salmoides

Ontogenetic changes in diet and jaw gape were compared between two indigenous populations of largemouth bass, Micropterus salmoides, to test the hypothesis that ecomorphology varies among broadly distributed fish populations. Two hundred seventy-two temperate (southwestern Michigan) M. salmoides and 265 subtropical (east-central Florida) conspecifics were analyzed for food habits and oral jaw gape height and width. Percent volumetric contributions of four functional prey categories (plankton, insect, crustacean, and fish) were compared among fish-size classes to determine if interval-specific differences in prey consumption existed between populations. Subtropical M. salmoides shifted from feeding on plankton and macroinvertebrates to fish by 20 mm standard length (SL); and stopped consuming plankton by 29 mm SL. Temperate largemouth bass did not become piscivorous until 37 mm SL and continued utilizing plankton up to 69 mm SL. Following the onset of piscivory, 100–260 mm SL subtropical largemouth bass began utilizing more crustaceans than fish. In contrast, temperate M. salmoides consistently fed on fish following the onset of piscivory. Variations in food habits were associated with differences in gape size between temperate and subtropical populations. Temperate largemouth bass had significantly larger gape height (ANCOVA: F = 103.4; df = 1,536; p < 0.001) and width (ANCOVA: F = 47.0; df = 1,536; p < 0.001) than subtropical bass. Although piscivory is a well-known feature of M. salmoides, the ontogeny of piscivory may vary between populations. We hypothesize that interpopulation differences in jaw gape may be related to variations in prey-resource utilization.

Ontogeny of diet changes in a tropical benthic carnivorous fish, Parupeneus barberinus (Mullidae): relationship between foraging behaviour, habitat use, jaw size, and prey selection

Parupeneus barberinus forages on benthic invertebrates using a wide range of foraging modes, including vigorous digging in the substratum, resulting in considerable disturbance to the benthos. Polychaetes were the most important prey item for all size classes, but fishes less than 120 mm total length consumed more small ostracods and nematodes than did larger fishes. Fishes greater than 120 mm total length consumed mostly bivalves, and fishes over 240 mm total length consumed mostly bivalves and crabs. A morphological examination of the feeding apparatus suggested that the size of important prey items consumed was determined by gape height and jaw width. Prey available to different size classes of fishes was determined by combining information on microhabitat use, foraging behaviours, and prey volumes in the substratum. Small fishes spent more time foraging on the reef flat and slope, compared with larger fishes that foraged mostly on the reef edge and base. In addition smaller fishes foraged mostly in the upper 2 cm of sediment, whereas larger fishes often foraged to depths of 10 cm. Selection ratios showed that different size classes of fishes selectively extracted different prey items from the substratum. Small fishes showed a preference for ostracods whereas large fishes selected for bivalves and crabs. Although polychaetes were the dominant prey item for all size classes, they were consistently selected against.

Size-Dependent Alewife Predation on Larval Walleyes in Laboratory Experiments

Abstract Alewives Alosa pseudoharengus can be significant predators on larval fish, and fishery managers must consider potential predation when developing stocking strategies. We conducted laboratory experiments to determine sizes of larval walleyes Stizostedion vitreum that alewives can capture. Adult alewives (122 ± 1.52 mm total length, TL) preyed intensely on larval walleyes from the time the larvae hatched until they reached about 16 mm TL. All larval walleyes smaller than this size were captured on the first strike by an alewife. Avoidance behavior by larval walleyes was first observed when they reached 16–19 mm TL, and although some survived the first strike, all were consumed by the end of 1 h. When larvae averaged about 25 and 30 mm TL, few were captured on the first strike, and 75% and 33%, respectively, were consumed within 1 h. When larvae were about 34 mm TL, only 5% were eaten by the end of 1 h. Alewife gape height and width were both about twice as large as the largest larval walleye body h...

Size of prey ingested by whitefish, Coregonus sp., larvae. Are Coregonus larvae gape‐limited predators?

The feeding ecology of whitefish, Coregonus, larvae in oligotrophic Lake Sarnen and eutrophic Lake Hallwil is presented, taking into consideration the size of the prey ingested in relation to the mouth size of larvae and to the availability of zooplankton.When larvae grow from 10 to 16 mm their mouth width increases from 500 to 1000 μm while their gape height, with the mouth open at 45 and 90°, increases from 200 to 400 μm and from 400 to 700 μm, respectively, whichever lake is considered.The differences found in the mean prey width between the two lakes arise in part from the composition of diets, and in part from differences in size of the available prey organisms. Whatever the origin of the larvae and the taxon we consider, larvae seem to ingest the most abundant prey size without selecting the biggest ones available even if, from a mechanical point of view, they would be able to ingest them.Taking into account the generally low density of zooplankton in the uppermost layer of the lake, we conclude that whitefish larvae eat in the manner such as to decrease the cost of prey capture, in terms of energy and time, by choosing the more abundant prey available.

100尺2枚仕立てトロール網の海上実験

For the purpose of obtaining some effective suggestions concerning the construction of the unigan trawl net, the working behaviors of a two piece net (Fig. 1:100 feet in head rope length) were examined on board the Ten'yo-maru (450GT, 1000HP)1), in succession to the previous report2) on the shooting and hauling operation. The results obtained are summarized as follows: 1) The gape height, HNM (m), of the 100-foot net was expressed as a fractional function of the towing speed, V (m/sec), by the following empitical formula (Fig. 2): HNM=2.2/V1.5. 2) The tension, TWP (ton), of warp at the boardside depended both the towing speed, V (m/sec) and the warp length, LWP (m), keeping the following empirical formula (Figs. 3 and 4): TWP=V0.9(1+0.0003 LWP). 3) When the towing speed was very slow, the shape of the warp curved sharply downwards. But, its curvature became less seveer in accordance with increase in the towing speed, and the curve approached very closely to a straight line when the towing speed was higher than about 1.5-1.75m/sec (Fig. 5). 4) The working depth, D (m), of the otter board was indicated, as a continued product of an exponential and a fractional function regard to the warp length, LWP (m), and the towing speed, V (m/sec), by following empirical formula (Fig. 6): D=27e0.0063LWPV-1.5. 5) The working depth of net was about the same with that of the otter board regardless of the towing speed. Hence, the shape of the hand rope (sweep line) during towing was assumed to have a horizontal straight line which is paralell to the sea-bed (Fig. 6).