Growth and abundance of Fundulus heteroclitus and Menidia menidia in estuaries of Waquoit Bay, Massachusetts exposed to different rates of nitrogen loading

To assess the effect of land-derived nitrogen loads on fish populations, we estimated the abundance and growth rate of Fundulus heteroclitus and Menidia menidia, the most common fish species found in estuaries of Waquoit Bay, from June through September in 1996, 1997 and 1998 in six estuaries subject to different rates of nitrogen loading. Abundance of F. heteroclitus was greater in estuaries with greater nitrogen loading rates, but growth rates of F. heteroclitus obtained from growth rings on scales and otoliths did not vary among estuaries. Abundance and growth rates of M. menidia did not change with increased nitrogen loading rates. Increased nitrogen load may increase abundance of some fish species within an assemblage but not others. Such bottom up controls are effective at producer trophic levels, but appear to be less pervasive with secondary consumers. Higher trophic levels, therefore, may only be loosely coupled to nutrient supply rates.     

Laboratory spawning cues in Menidia beryllina,and M. peninsulae (Pisces,Atherinidae) with notes on survival and growth of larvae at different salinities

Synopsis Spawning patterns of inland silversides, Menidia beryllina, and tidewater silversides, Menidia peninsulae, were examined in the laboratory under several combinations of ‘tidal’ and diel light cycle cues. M. beryllina showed a high frequency of spawning throughout the day when held under constant conditions (24L: OD, current velocity 8 cm sec⁻¹) and when ‘tidal’ and diel light cycles were presented singly or in combination. In contrast, M. peninsulae demonstrated a high frequency of spawning only when presented a combination of ‘tidal’ and diel light cycle cues and spawned predominantly at night. Menidia beryllina embryos were euryhaline. Hatching ranged from 73 to 78% at salinities of 5,15 and 30‰ M. peninsulae embryos showed an inverse relationship between the percentage hatch and the incubation salinity, 90% at 5‰ and only 65% at 30‰ Survival and growth of larval M. beryllina from the day of hatching through 16 days old was optimal at 15‰ Although survival of M. peninsulae larvae was optimal at 30%, no trend was apparent in growth of larvae held for 16 days at 5, 15 or 30‰ salinity. Contribution No. 508 from the Gulf Breeze Environmental Research Laboratory     

Rotifers,cladocerans and planktivorous fish: What are the major interactions?

Results of ten outdoor tank experiments on the effects of two kinds of planktivorous fish (Dorosoma cepedianum and Menidia beryllina) on phytoplankton and zooplankton were reviewed for evidence of rotifer-cladoceran interactions. Correlation and multiple regression analyses of the responses of populations of six rotifer species in ten experiments were examined for evidence of interference competition with daphnids, resource exploitation, or invertebrate predation by crustaceans associated with daphnid-dominated communities. A separate enclosure experiment was conducted to assess the potential for short-term effects of dense daphnid populations on four rotifer populations. Indirect resource-mediated effects on rotifer populations associated with manipulations of planktivorous fish were more numerous and consistently expressed than direct negative effects of cladocerans or copepods on rotifer populations.     

Morphological and behavioral antipredatory adaptations of decapod zoeae

Summary Zoeae of some species of estuarine decapods are retained in the estuary throughout development while others are exported into nearshore coastal waters. The horizontal migrations of decapod zoeae to coastal waters may have evolved to reduce the probability of encountering planktivorous fishes which are most abundant in the estuary. If so, then the morphological vulnerability of zoeae to fish predation should be inversely related to the number of predators occurring where they develop. Six species of estuarine decapod zoeae were offered to Menidia menidia and Fundulus heteroclitus. The behavioral interactions were observed to determine the prey's vulnerability to predation, and the mode of operation and relative effectiveness of their defenses. Feeding trials and behavioral observations both demonstrated that M. menidia 6–16 mm long preferred Uca minax and Callinectes sapidus zoeae, which are exported from the estuary, to Rhithropanopeus harrisii, Sesarma reticulatum and Palaemonetes pugio, which are retained within estuaries. Pinnotheres ostreum zoeae develop in the lower estuary and fish demonstrated an intermediate preference for the zoeae. Menidia menidia 20–40 mm long showed similar preferences for R. harrisii S. reticulatum, P. ostreum and U. minax as did small silversides. Large-mouthed demersal fish, Fundulus heteroclitus 6–10 mm long, also preferred U. minax to R. harrisii, but more readily preyed on zoeae than did M. menidia. The exported species of zoeae have shorter spines and smaller bodies than do retained zoeae, except P. ostreum which is small, spineless and passively sinks when attacked by fish. Other retained species of zoeae also have postcontact behavioral defenses which enhance the effectiveness of their morphological defenses. Zoeae do not evade attacks by fishes, but fishes quickly learned to avoid zoeae, which increases the effectiveness of the zoeae's antipredatory adaptations.     

The feeding habits of Sargochromis Codringtonii in Lake Kariba, Zimbabwe

The food of adult Sargochromis codringtonii in Lake Kariba was dominated by prosobranch snails with pulmonate snails forming an insignificant dietary component. S. codringtonii does not show particular preference for any snail species, feeding on those species that are readily available. Morphometrical analysis of the feeding structures of S. codringtonii showed that the fish is well adapted to handling a molluscivorous diet. The pharyngeal jaws with their flat crowned teeth are well adapted to crushing molluscs. The daily food consumption rate of S. codringtonii was estimated over different seasons. Estimates of food consumption indicate that the fish consumed 13.96% of their dry body weight per day in summer falling to 4.46% in winter. The low consumption of pulmonate snails suggests that S. codringtonii can not be a successful biological control agent against the vectors of schistosomiasis.     

Adaptive variation in energy acquisition and allocation among latitudinal populations of the Atlantic silverside

Understanding the evolution of growth rate requires knowledge of the physiology of growth. This study explored the physiological basis of countergradient variation (CnGV) in somatic growth across latitudinal populations of the Atlantic silverside, Menidia menidia. Energetics of northern (Nova Scotia, Canada) and southern (South Carolina, USA) genotypes were compared across resource levels, temperatures, and fish sizes to identify trade-offs to rapid growth. Offered unlimited resources, genotypes differed in both energy acquisition and allocation. Food consumption, growth, and efficiency of northern genotypes were consistently higher than in southern genotypes, across temperatures and body sizes. Feeding metabolism (specific dynamic action; SDA) was proportional to meal size, differing between genotypes to the extent that food consumption differed. Given limited resources, northern and southern genotypes displayed similar growth, efficiency, routine activity, and SDA across temperatures and fish sizes. Routine metabolism was equal at 17°C and 22°C, yet was significantly higher in northern fish at 28°C. Growth rates in M. menidia do not appear to trade off across environments or body sizes, i.e., at no temperature, ration, or size do southern fish outgrow northern conspecifics. Nor does submaximal growth result from increased costs of maintenance, tissue synthesis, or routine activity. Based on our findings, we propose that CnGV consumption and growth in M. menidia likely result from trade-offs with other energetic components, namely sustained and burst swimming. Received: 26 January 1999 / Accepted: 14 September 1999     

Bioaccumulation of Trace Mercury in Trophic Levels of Benthic, Benthopelagic, Pelagic Fish Species, and Sea Birds from Arvand River, Iran

In this study, concentration of mercury was determined in the trophic levels of benthic, benthopelagic, pelagic fish species, and river birds from Arvand River, located in the Khuzestan province in the lowlands of southwestern Iran at the head of the Persian Gulf. The order of mercury concentrations in tissues of the fish species was as follows: liver>gill>muscle and in tissues of the kingfisher species was as follows: feather>liver>kidney>muscle. Therefore, liver in fish and feather in kingfisher exhibited higher mercury concentration than the other tissues. There was a positive correlation between mercury concentrations in fish and kingfisher species with size of its food items. We expected to see higher mercury levels in tissues of female species because they are larger and can eat larger food items. The results of this study show that the highest mean mercury level were found in the kingfisher (Anas crecca), followed by benthic (Epinephelus diacanthus), benthopelagic (Chanos chanos), and pelagic fish (Strongylura strongylura). Mean value of mercury in fish species, S. strongylura were (0.61 μg g^?1 dry weight), C. chanos (0.45 μg g^?1 dry weight), E. diacanthus (0.87 μg g^?1 dry weight), and in kingfisher species A. crecca was (2.64 μg g^?1 dry weight). Significant correlation between mercury concentration in fish and kingfisher may be related to high variability of mercury in the fish.     

Potential effect of floating dock communities on a South Carolina estuary

A 4-yr study on the motile epibionts colonizing hydroids on a floating dock in a South Carolina estuary indicated that the motile epibiotic community may impact benthic and nektonic community processes. Permanent members of the epibiotic community were preyed upon by fish, especially when the Atlantic silverside (Menidia menidia (L.)) became reproductively active and deposited eggs among the hydroids during the spring. When fish predation reduced the number of amphipods within the epibiotic community, crab megalopa colonized the hydroids. Crabs developed into juveniles and emigrated from the epibiotic community after 6–8 wk residence. The community on floating docks may be an important food source for some nekton and may recruit some benthic invertebrates.Although preferential larval settlement and fish predation are intuitive processes associated with floating dock communities, insufficient research has addressed the relationships in soft-bottom areas devoid of naturally occurring hard substrata.     

Molecular and morphological divergence in the inland silverside (<Emphasis Type="Italic">Menidia beryllina</Emphasis>) along a freshwater-estuarine interface

Populations that vary across ecological gradients or that have invaded novel habitats are important to elucidate the association between adaptive divergence and gene flow, factors that may play an important role in speciation of silverside fishes. The inland silverside, Menidia beryllina, is an ideal organism for this kind of research, displaying a great diversity in morphology among freshwater and coastal brackish habitats. Using a combination of geometric morphometrics and mitochondrial (mt) DNA, we investigated patterns of variation within and among the nominal freshwater Menidia audens and coastal M. beryllina, spanning the transition from freshwater to tidally influenced semi-brackish waters of the lower Mississippi River to brackish waters of the Lake Pontchartrain estuary. Although we found no evidence for a phylogenetic split between M. audens and M. beryllina, our results indicate that significant genetic divergence corresponds with body shape differences among the two, with a clear distinction at the interface of freshwater and brackish water. Patterns in mtDNA suggest that freshwater populations referred to as M. audens are of recent origin with evidence for habitat-based divergence compared to coastal populations referred to as M. beryllina. Our findings add to a growing body of evidence that ecological shifts, following colonization of novel habitats, may promote rapid adaptive divergence and reduced gene flow among silverside populations in adjacent environmental regimes.     

Effects of food density and temperature on feeding and growth of young inland silversides (Menidia beryllina)

Food consumption and growth rates of 7–28-day-old Menidia beryllina were measured in response to natural ranges of temperature and prey availability. Feeding level, temperature and age all had significant effects on growth rate, although the effect of feeding level explained most of the variance. Feeding level also had a significant effect on gross growth efficiency, but temperature and age did not. Absolute growth rates (mg per day) increased dramatically with temperature, feeding level, and age. Variability in growth was greatest for fish feeding at the lowest feeding level. For a given fish weight, temperature had a positive effect on consumption rate, and maximum consumption ( C _max) of any treatment combination reached 75% body weight per day. Maximum growth rate was estimated at 24.6% body weight per day, and gross growth efficiency reached an estimated maximum of 0.375 at an ingestion rate of 25% body weight per day. Starved larvae lost on average 5.4% body weight per day and larvae required 6.4% body weight food consumption per day for maintenance. Multiple regressions of feeding level, temperature, and age/size on instantaneous growth rates indicated that increases in temperature increased maintenance requirements and required that fish consume a greater proportion of C _max to attain maximum growth. Growth rates decreased with increases in temperature for fish eating a specific weight of food.     

Daily rhythms of swimming activity,synchronization to different feeding times and effects on anesthesia practice in an Amazon fish species (Colossoma macropomum)

ABSTRACT

This study aimed to investigate the existence of day-night differences in the time for anesthesia and recovery in tambaqui exposed to the anesthetic eugenol and the influence of feeding time. Thus, we evaluated: (1) swimming activity; (2) food anticipatory activity (FAA) as a synchronizer of swimming activity and change to susceptibility to anesthetic; and (3) the effects of diurnal/nocturnal anesthesia exposure of fish feeding in the mid-light phase: 12:00 h (ML) and fish feeding in the mid-dark phase: 00:00 h (MD). Our findings revealed strictly nocturnal activity for tambaqui (94.2%), known as diurnal fish to date. Moreover, FAA was observed in tambaqui fed at MD, which showed a sustained increase in activity that began 2 h before feeding time and lasted until feeding. In contrast, no FAA was observed in fish fed at ML. Regarding anesthesia by day or night, the tambaqui treated with eugenol exhibited no difference in induction time. However, differences were observed in recovery times, with fish anesthetized at day recovering in 1–2 min and fish anesthetized at night recovering in 5–7 min. In short, our findings revealed for the first time the nocturnal behavior of tambaqui. These results indicated that recovery by day/night by eugenol in tambaqui has a strong dependence of behavioral patterns and the time of day.     

Field and laboratory observations of spawning periodicity and behavior of a northern population of the Atlantic silverside,Menidia menidia (Pisces: Atherinidae)

Synopsis The spawning periodicity and mating behavior of a northern population of the Atlantic silverside, Menidia menidia, was studied both in the field (Salem Harbor, Massachusetts) and in laboratory experiments. Spawning in the field coincided with new and full moons. Under conditions of unlimited food availability in artificial outdoor pools that received natural celestial illumination or in indoor aquaria receiving no evening illumination, spawning occurred every 1–3 days rather than fortnightly. These results suggest that tidal influences are a primary factor that synchronizes spawning in M. menidia. Published reports of diel time of spawning in both marine and freshwater species of Menidia indicate a generic tendency to spawn during mid-morning. This pattern coupled with high tides that cover suitable spawning substrates during mid-morning only at fortnightly intervals could account for the semilunar spawning cycle in marine populations of Menidia. Although general characteristics of the reproductive ecology of M. menidia in Massachusetts were similar to a thoroughly studied population in South Carolina, several differences were evident. The breeding season was shorter in Massachusetts (late; April–June), and occurred over a much lower range of temperatures (9–21°C). Intensity and frequency of spawning was correlated with the height of high tide. Eggs were deposited only on mats of intertidal, filamentous algae rather than on roots or stems of Spartina alterniflora or other intertidal vegetation. Promiscuous spawning occurred in small, highly male-dominated groups of fish, primarily after the tide had begun to recede, and often at the water's edge. No predation on spawning adults was observed but the mummichog, Fundulus heteroclitus, fed daily on developing embryos during high tide. Contribution No. 81, Massachusetts Cooperative Fisheries Research Unit, Contribution No. 349, Marine Sciences Research Center, State University of New York at Stony Brook.     

Coevolution of foraging behavior with intrinsic growth rate: risk-taking in naturally and artificially selected growth genotypes of <Emphasis Type="Italic">Menidia menidia</Emphasis>

Although there is accumulating evidence of growth-rate optimization by natural selection, the coevolution of growth rate and risk-taking behavior has not been sufficiently documented. The Atlantic silverside fish, Menidia menidia, displays countergradient variation in growth across a latitudinal gradient: genotypes from Nova Scotia (NS), for example, grow in length twofold faster than those from South Carolina (SC). Past work has established that fast growth is adaptive in northern climates, but the trade-off is poorer swimming performance and higher susceptibility to predators. We compared escape behavior and willingness to forage under threat of predation among growth genotypes reared and tested under common-garden conditions. When chased with a predator model, NS fish occupied shelter more quickly than SC fish. When food was supplied after a chase, NS fish reemerged from the shelter much more quickly than SC fish and immediately commenced feeding, whereas many SC fish displayed timid behavior and did not feed. When food was absent following a chase, however, NS fish remained in the shelter longer than did SC fish and both displayed timid behavior. Hence, the fast-growing NS genotype was bolder than SC fish in the presence of food, but shyer in the absence of food. These behaviors are adaptive given the physiological constraints intrinsic to each genotype. Experiments on captive populations of silversides that had been artificially selected for fast or slow growth confirmed that foraging behavior is genetically correlated with intrinsic growth rate, although in these trials the fast-growth genotype was always more bold, regardless of food availability, as would be expected in the absence of predators. We conclude that risk-taking foraging behavior coevolves adaptively with intrinsic growth rate in M. menidia.     

The role of floods in the lives of fish-eating birds: predator loss or benefit?

Floods accompanied by high flow and high water turbidity are usually believed to cause problems to fish-eating birds and mammals searching visually for their prey. In the present study the diets of breeding kingfishers were studied during the normal river situation and during a long-lasting flood event with respect to diet composition, size of fish prey and food diversity index. During the normal situation (flow 1.75 m³ s^?1, Secchi disc depth 0.5–1 m), the diet of a kingfisher was dominated by benthic fish species (52.9% by numbers, 63.9% by weight), the average size of fish taken was 6.5 cm L _T and 3.0 g and the food diversity index reached its lowest value (1.57). In contrast, during the long-lasting flood event (flow 5–28 m³ s^?1, Secchi disc depth 0.03–0.4 m) the diet of the kingfisher was dominated by sub-surface fish species (72.4% by numbers, 76.1% by weight) and both the average size of fish taken (7.4 cm L _T and 3.7 g) and the food diversity index (1.83) increased significantly. The birds provided their nestlings with lower numbers of fish of larger sizes, which resulted in very similar weights of the young birds prior to fledging when the flood and normal situations were compared. This study provides evidence that in different foraging conditions the kingfishers adopt different foraging strategies to maintain their high breeding success.     

Exploration of spatial and temporal patterns of fish diversity and composition in a tropical estuarine system of Mexico

There are many studies of fish assemblages intemperate systems, but little informationexists about tropical estuarine systems,particularly in the Western Atlantic Regionincluding Mexico. We investigated the fishcommunity structure in the Pueblo Viejo lagoon,Veracruz. Biological samples were collectedmonthly for one year at six sites: three withdense stands of Ruppia maritima and threesites without submerged vegetation. For eachsample, temperature, salinity, dissolvedoxygen, turbidity and depth were analyzed. Theunbiased Simpson diversity index, CanonicalCorrespondence Analysis (CCA) and DiscriminantAnalysis (DA) were used to analyze the fishcommunity. Environmental abiotic variablestended to show significant temporal, but notspatial differences. Fish diversity showedonly a weak significant correlation with watertemperature, and relatively strong diversitypeaks, from June to August and April, wererelated to production peaks in the system. Thetwo first axes of CCA accounted for 65% of thespecies-environment biplot variance, whichsuggested that monthly changes of salinity,turbidity and precipitation, and presence orabsence of submerged vegetation, were the mostimportant environmental variables indetermining the observed variability in fishcommunity composition. Further, fish diversitywas significantly different between habitatswith and those without the presence ofsubmerged vegetation (P < 0.02). A DA showedsignificant differences (P < 0.03) in fishcommunity composition between both kinds ofhabitats, with Lagodon rhomboides,Mugil curema and Menidia beryllina(substantially more abundant in habitat withsubmerged vegetation) as the most importantspecies in the discrimination of spatial fishcomposition. Considering both habitatscombined, fewer differences were observed infish diversity and community compositionbetween rainy and dry seasons. Fish communitycomposition showed greater similarity betweenseasons than between habitats, despite the factthat environmental abiotic variables showed aninverse pattern, suggesting that site factors,such as the presence of submerged vegetation,play a more important role in the maintenanceof fish community patterns than those relatedto temporal influence.     

Influence of body weight/age on the food intake,growth and conversion efficiency of Gambusia affinis

On a restricted food supply of Tubifex tubifex (2h/day) the fish consumed only 69.5 mg dry food/g live fish/day and grew only to 25.6 mg live body weight by the 30th day of it's age, while those fed ad libitum consumed 94.7 mg/g/day and attained a body weight of 125.9 mg live weight by the 30th day. Conversion efficiency (K₁) averaged to 8.3% in the former and 14.1% in the latter series, throughout the 30 day experimental period.Feeding rate of the test series fed individually and at ad libitum, decreased from 780 mg dry food/g dry fish/day for a fish weighing 4 mg dry weight to 180 mg/g/ day for one weighing 41 mg, exhibiting a log-log relationship to the body weight (Y = 2.20140–0.5639 X). Growth rate also decreased as a log-log function with increasing body weight (Y = 1.2309–0.4384 X).     

Non‐lethal predator effects on the performance of a native and an exotic crayfish species

1. I tested the hypothesis that the potential for non‐lethal effects of predators are more important for overall performance of the fast‐growing exotic signal crayfish (Pacifastacus leniusculus Dana) than for the slower growing native noble crayfish (Astacus astacus L.). I further tested if omnivorous crayfish switched to feed on less risky food sources in the presence of predators, a behaviour that could reduce the feeding costs associated with predator avoidance. 2. In a 2 month long outdoor pool experiment, I measured behaviour, survival, cheliped loss, growth, and food consumption in juvenile noble or signal crayfish in pools with either a caged predatory dragonfly larvae (Aeshna sp.), a planktivorous fish that do not feed on crayfish (sunbleak, Leucaspius delineatus Heckel), or predator‐free controls. Crayfish had access to multiple food sources: live zooplankton, detritus and periphyton. Frozen chironomid larvae were also supplied ad libitum outside crayfish refuges, simulating food in a risky habitat. 3. Crayfish were mainly active during hours of darkness, with signal crayfish spending significantly more time outside refuges than noble crayfish. The proportion of crayfish outside refuges varied between crayfish species, time and predator treatment, with signal crayfish spending more time in refuges at night in the presence of fish. 4. Survival in noble crayfish was higher than in signal crayfish, and signal crayfish had a higher frequency of lost chelipeds, indicating a high level of intraspecific interactions. Crayfish survival was not affected by the presence of predators. 5. Gut‐contents analysis and stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) indicated that the two crayfish species had similar food preferences, and that crayfish received most of their energy from feeding on invertebrates (e.g. chironomid larvae), although detritus was the most frequent food item in their guts. Signal crayfish guts were more full than those of noble crayfish, but signal crayfish in pools with fish contained significantly less food and fewer had consumed chironomids compared with predator‐free controls. Length increase of signal crayfish (35%) was significantly higher than of noble crayfish (20%), but signal crayfish in pools with fish grew less than in control pools. 6. This short‐term study indicates that fish species that do not pose a lethal threat to an organism may indirectly cause reductions in growth by affecting behaviour and feeding. This may occur even though prey are omnivorous and have access to and consume multiple food sources. These non‐lethal effects of predators are expected to be particularly important in exotic crayfish species that show a general response to fish, have high individual growth rates, and when their feeding on the most profitable food source is reduced.     

Early Responses of Fishes and Crustaceans to Restoration of a Tidally Restricted New England Salt Marsh

Nekton (fishes and decapod crustaceans) is an abundant and productive faunal component of salt marshes, yet nekton responses to tidal manipulations of New England salt marshes remain unclear. This study examined nekton use of a tidally restricted salt marsh in Narragansett, Rhode Island relative to an unrestricted marsh during summer. In addition, a before‐after‐control‐impact design was used to examine early responses of nekton to the reintroduction of natural tidal flushing. Species richness and densities of Cyprinodon variegatus, Lucania parva, Menidia beryllina, and Palaemonetes pugio were higher in the restricted marsh compared with the unrestricted marsh. The unrestricted marsh supported higher densities of Menidia menidia and Fundulus majalis. Mean lengths of Carcinus maenas and P. pugio were greater in the restricted marsh. Tidal restoration resulted in increased tidal flushing, salinity, and water depth in the restricted marsh. Densities of Fundulus heteroclitus, F. majalis, and Callinectes sapidus were higher after 2 years of restoration. Density of L. parva decreased after restoration, probably in response to a loss of macroalgal habitat. Species richness also decreased after 2 years, from 20.9 species when the marsh was restricted to 13.0 species. Total nekton density did not change with restoration, but shifts in community composition were evident. In this study restoration induced rapid changes in the composition, density, size, and distribution of nekton species, but additional monitoring is necessary to quantify longer‐term effects of salt marsh restoration on nekton.     

Intra- and inter-specific variation in buoyancy of some estuarine fishes

Synopsis Buoyancy was measured on eight species of estuarine fishes that were caught in 1 m depth or less. Mean buoyancies of the physoclists Fundulus heteroclitus, F. majalis, Cyprinodon variegatus and Leiostomus xanthurus were similar and ranged from –6.5 to –18.0 kiloPascals below atmospheric pressure at sea level. Menidia menidia and Pomatomus saltatrix measured –36.6 and –46.1 kPa, respectively. Two physostomes, Brevoortia tyrannus and Anchoa mitchilli, measured + 2.9 and –23.5 kPa, respectively, but the latter probably releases air when handled.The four most buoyant physoclist species live near the bottom in areas that receive daily tide induced currents. Negative buoyancy probably functions in them as in stream dwelling minnows and salmonids, which respond to currents by decreasing their buoyancy. The pronounced negative buoyancy of M. menidia may be a response to a preference for habitat where the currents are stronger, P. saltatrix, which can secrete gas into the swim bladder at the fastest rate known for any fish, combines high secretion (and resorption) rates with marked negative buoyancy. This enables it to quickly change depths over a wide vertical range, without overexpanding the swim bladder to cause positive buoyancy.