Crab Megalopae Research Articles

Response of mud crab (Panopeus herbstii) megalopae to cues from adult habitat

During the summer of 1995, we investigated the response of mud crab megalopae (Panopeus herbstii Milne-Edwards) to cues associated with adult habitat. Natural rock/shell substratum and the rock-associated seaweed, Fucus vesiculosus L., both induced metamorphosis of competent megalopae; natural sand substratum did not induce metamorphosis. Structural mimics of preferred substrata induced metamorphosis only when covered with natural biofilm. Clean mimics did not induce metamorphosis. Water-soluble exudates from preferred substrata showed weak induction of metamorphosis. Exudates from adult P. herbstii elicited a similar weak response. Exudates from another species of crab and from two fish predators did not induce metamorphosis. We conclude that water-soluble cues associated with major components of preferred adult habitat induce the metamorphic molt of P. herbstii megalopae. The three-dimensional structure of the substratum is not important in induction of metamorphosis, but the biofilm associated with preferred substratum plays a critical role. It is not clear whether the biofilm produces the water-soluble cue or simply provides a tactile stimulus that works in conjunction with the soluble cue.

Responses of estuarine crab megalopae to pressure, salinity and light: Implications for flood-tide transport

The megalopal larval stage of many estuarine brachyuran crabs appears to return to adult habitats by undergoing rhythmic vertical migrations which result in saltatory up-estuary transport on flood tides. Larval ascent into the water column during rising tides may be cued by changing hydrologic variables. To test this hypothesis, we investigated the responses of field-caught megalopae of the blue crab Callinectes sapidus and the fiddler crab Uca spp. to constant rates of pressure and salinity change under laboratory conditions. For both genera, pressure changes resulted in increased movement (barokinesis) and upward migration in the test chamber, with C. sapidus megalopae having a lower response threshold (2.8×10-2 mbar s-1) than Uca spp. larvae (5×10-2 mbar s-1). Similarly, larvae ascended in response to increasing salinity, with C. sapidus larvae being more sensitive. Larvae were negatively phototactic and failed to respond to pressure increases at light levels above 1.0×1015 and 1.0×1013 photons m-2 s-1 for C. sapidus and Uca spp. megalopae, respectively. Such responses are thought to explain the low abundances of larvae in the water column during daytime flood tides. Nevertheless, threshold sensitivities to increasing pressure for both genera were above levels experienced during floodtide conditions in the field. Similarly, it is unlikely that increasing salinity is sufficient to induce ascent in Uca spp. postlarvae. However, rates of salinity increase during midflood tide typically reach levels necessary to induce an ascent in C. sapidus megalopae. These results are consistent with the hypothesis that fiddler crab megalopae utilize an endogenous activity rhythm for flood-tide transport, while blue crab megalopae rely upon external cues, especially salinity changes, to time their sojourns in the water column.

Pre‐ And Post‐Settlement Determinants of Estuarine Dungeness Crab Recruitment

Abundance of early juvenile Dungeness crab (Cancer magister) is dramatically higher in intertidal shell habitats compared to mud habitats in several coastal estuaries of the Pacific Northwest. To define the mechanisms underlying this habitat—specific pattern in abundance, we concurrently examined four components of recruitment to intertidal shell and mud habitats at two locations within the Grays Harbor estuary (Washington, USA): (1) water column supply of crab megalopae (postlarvae); (2) settlement patterns of crab megalopae 8 h after settlement substrates were deployed; (3) density of first benthic juvenile instars (J1) 48 h after deployment of such substrates; and (4) density of early juvenile crab in shell and mud habitats over a 4—mo period. We also describe the physical processes likely to be influencing postlarval supply within Grays Harbor, and take advantage of natural variation in postlarval supply between two locations, in combination with a predator exclusion experiment, to define the relative importance of postlarval supply vs. post—settlement survival in regulating population size of juvenile crab in certain intertidal habitats. Water column postlarval supply (measured with plankton and neuston nets, and artificial settlement substrates) in terms of both megalopal density (number per cubic metre) and flux (number per hour) was significantly higher in the southern part of the estuary vs. the northern part during a week—long settlement pulse. Our field observations and measurements suggest that spatial variation in postlarval supply was due to local differneces in wind—driven surface currents, since tidal current speeds in the two locations were similar. More—over, there was no correlation between current speed and flux of megalopae over the bottom. There was generally no difference in postlarval supply between shell and mud habitats. Our experimental results further indicate that: (1) the abundance of recently settled crab megalopae in 0.25 m2 settlement trays was significantly higher in shell than in mud habitats, irrespective of whether the trays were placed in 3—5 ha of shell vs. mud; (2) there was a positive and significant correlation between postlarval supply and density of megalopae in shell and mud habitats; and (3) there was a positive and significant correlation between postlarval supply and density of J1 instars only in habitats where specific predators were excluded. Once the number of J1 instars at both geographical locations was reduced to similar levels, equivalent but steadily decreasing densities persisted throughout the summer growing season. The decoupling of settlement patterns and density of J1 instars took place within our 48—h sampling interval. Thus, future attempts to examine the correspondence between larval supply and post—settlement abundance of marine benthic species with planktonic larvae should do so at extremely small temporal scales or a critical life history phase may be overlooked. The results from this study demonstrate that substrate selection can affect distribution of juvenile crab, and that predation (including cannibalism) is a key factor regulating local population size of early juvenile crabs in intertidal habitats where postlarval supply is relatively high.

Orientated swimming by megalopae of several eastern North Pacific crab species and its potential role in their onshore migration

The megalopal stage of most near shore and intertidal crabs must return to the coast to complete their development. Crab megalopae are strong swimmers and if they swam consistently shoreward they could conceivably swim back to shore. This hypothesis was tested for megalopae of Pachygrapsus crassipes (Randall), Lophopanopeus bellus bellus (Stimpson, 1860), Cancer oregonensis (Dana), and C. gracilis (Dana) by investigating their swimming orientation when housed in a transparent container with a view of the underwater illumination or swimming freely in the sea. In the transparent container, megalopae tended to swim in the direction of the sun's bearing. Free swimming megalopae swam straight courses and displayed significant preferred swimming directions. Pachygrapsus crassipes and L. bellus bellus megalopae swam at the sea surface and parallel to the current direction. Free swimming C. oregonensis and C. gracilis swam at about 3 to 5 m depth and in the direction of the sun's bearing. Megalopae of Pachygrapsus crassipes and C. oregonensis were observed on 4 and 3 days respectively and they did not preferentially swim in a shoreward direction. These results suggest that they do not migrate back to shore by swimming. The orientated swimming may, however, assist the megalopae in returning to shore or locating a settlement site. Swimming with the surface current might help megalopae to migrate shoreward in the convergence zone over internal waves or Langmuir circulation cells. Swimming in the direction of the sun's bearing might represent a search behavior for a benthic settlement site.

Settlement of brachyuran megalopae in Delaware Bay:an analysis of time series data

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 125:67-76 (1995) - doi:10.3354/meps125067 Settlement of brachyuran megalopae in Delaware Bay: an analysis of time series data Jones MB, Epifanio CE Crab megalopae were collected daily in a secondary estuary near the mouth of Delaware Bay during 4 annual settlement seasons (1989 to 1992). The sampling site consisted of a raft from which 4 replicate collectors were deployed vertically at the water column surface. Each replicate collector was constructed from a segment of 20 cm diameter PVC pipe. The pipe was covered with a removable sheath of loosely interwoven strands of polypropylene upon which the megalopae settled. We assumed that settlement on the collectors was proportional to settlement on natural substrata near the sampling site each day. The 3 families Ocypodidae (Uca spp.), Portunidae (Callinectes sapidus), and Xanthidae comprised the vast majority of the megalopae collected. Daily settlement events occurred in clusters. Auto-correlation analysis showed decorrelation times ranging from 2 to 3 d for C. sapidus and from 1 to 2 d for xanthids and Uca spp. These define the durations of typical settlement episodes for each taxon. Xanthids and Uca spp. began to settle in mid summer and continued into early autumn. Major settlement of C. sapidus never occurred until late August, and the intensity of settlement usually decreased by late October. Results of G-tests showed no effect of lunar phase on settlement events, regardless of year or taxon. Likewise, binomial goodness-of-fit tests showed no significant effect of the spring/neap cycle on settlement. However, the observed frequency of settlement events for C. sapidus was significantly greater than expected for southward wind events in all 4 yr, while there was no consistent association between winds and settlement of Uca spp. or xanthids. Of the 3 taxa studied, only C. sapidus relies on wind-driven processes for transport to settlement sites. Transport of C. sapidus megalopae back into the estuary requires the coincident occurrence of a southward, alongshore wind event and a nearby patch of megalopae. The stochastic nature of this co-occurrence explains the observed temporal variations in settlement of C. sapidus in the estuary. Recruitment . Larval transport . Wind-driven . Megalopae . Crabs Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 125. Publication date: September 14, 1995 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1995 Inter-Research.

Trophic relationships and seasonal utilization of salt-marsh creeks by zooplanktivorous fishes

In a high salinity estuary at North Inlet, South Carolina, co-occurrence and possible competition among adults of four dominant zooplanktivorous fishes were minimized by seasonal adjustments in lateral and vertical distributions as well as in dietary preferences. In winter, Atlantic silversides, Menidia menidia, occupied the entire water column while other planktivores were rare or absent from the estuary, and they consumed large prey such as mysid shrimps and fish larvae. An immigration of bay anchovies, Anchoa mitchilli, in the spring resulted in a redistribution of species with Atlantic silversides shifting to the surface waters and bay anchovies dominating the lower half of the water column. Both fishes consumed mostly copepods in the spring, but each favored a different species. There was little similarity in the large prey items consumed by the two fishes. Striped anchovies, Anchoa hepsetus, arrived in mid-summer and were most abundant at the surface while bay anchovies continued to dominate the bottom waters. Atlantic silversides were rare in all summer collections. The diets of the two anchovies were similar, but vertical separation during the period of maximum zooplankton abundance probably minimized competition. Rough silversides, Membras martinica, which were obligate surface dwellers, shared the upper water column with striped anchovies, but the two species had very different diets during their period of co-occurrence. Although seasonal changes in fish diets reflected shifts in zooplankton composition and all fishes consumed a variety of prey types, preferences for some prey taxa and total avoidance of others were indicated. Electivity indices indicated an especially strong selection for fiddler crab megalopae by all fishes in the summer and fall. All fishes, except rough silversides, which fed almost exclusively on copepods and crab zoeae, consumed large prey items when they were available. Fine scale partitioning of the food resources was apparent in the selection of different copepod and insect species by the fishes. Spatial and temporal separation in the distribution and/or dietary preferences of the zooplanktivores fishes probably reduces the potential for resource competition. Given the high abundances and selectivity of the planktivores, significant impacts on some zooplankton populations probably result.

Photoresponses of crab megalopae in offshore and estuarine waters: Implications for transport

This study tested the hypothesis that fiddler crab ( Uca spp.) and blue crab [ Callinectes sapidus (Rathbun)] megalopae have separate behaviors in offshore and estuarine waters that are evoked by chemical cues. Past field studies indicate that blue crab megalopae are frequently in the neuston during the day offshore, while fiddler crab megalopae are at middle depths. Both species are rarely collected in the water column during the day in estuaries but are abundant during rising tides at night. Since photoresponses contribute to the day distributions of megalopae, we hypothesize that photobehavior changes dramatically between offshore and estuarine areas and used this as an assay for the effects of water-borne chemical cues. Photoresponses were tested in a light field that mimics the underwater angular light distribution and were quantified by measuring the proportion of megalopae swimming in the upper 1/3 of a water column. For blue crab megalopae collected offshore, swimming was significantly greater in offshore than in estuarine water and was suppressed in estuarine water at high light intensities. Thus, photoresponses contribute to depth maintenance near the surface during the day which is important for onshore transport by wind-generated surface currents. When megalopae were collected in an estuary, both species had greater swimming in offshore water than in estuarine water at all light intensities and in darkness. Thus, the absence of megalopae in the water column during the day in estuaries results from light suppression of swimming. Since the water only differed in chemical composition, the changes in photoresponses were induced by chemical cues. Tests with aged estuarine water indicated the chemical cues were relatively stable or initially present at high concentrations. These results support the hypothesis and suggest that megalopae have separate behaviors in coastal/shelf areas and in estuaries, which are induced by chemical cues in offshore and estuarine waters.

Swimming Behavior of Dungeness Crab, Cancer magister Dana, Megalopae in Still and Moving Water

In the Grays Harbor estuary, juvenile Dungeness crab (Cancer magister Dana) are found at higher densities in epibenthic shell deposits compared to open mud flat. Differences in predation rate between habitats have been suggested to be due to habitat preference and differential survival. Megalopae preferred shell over open space in still-water conditions. However, it is not known whether megalopae are able to select shell in flowing water since larval preference is known to differ between still and flowing water. Here we report the first experimental study of swimming behavior of Dungeness crab megalopae in a range of current velocities (0–40 cm s−1) equivalent to natural flow in Grays Harbor estuary. Experiments were conducted in daylight using a recirculating flume. Megalopae swimming speeds ranged from 8.5 cm s−1 (8 body lengths s−1) in still water to 44.8 cm s−1 (44 body lengths s−1) at flow speeds of 40 cm s−1, Neither swimming behavior nor sheltering behavior in shells showed any flow-related pattern. Megalopae spent a large proportion of time swimming against the current and made headway upstream against all current velocities tested. The results suggest that Dungeness crab megalopae are able to maneuver and actively search for settlement sites under current velocities found in natural habitats, including intertidal shell deposits, and support the hypothesis of active selection of shell by megalopae.

Endogenous swimming rhythms in estuarine crab megalopae: implications for flood-tide transport

Up-estuary migration of crab larvae to adult habitats is thought to be accomplished by selective tidal transport in which late-stage larvae enter the water column on flood tides and remain on or near the bottom on ebb tides. This study measured endogenous rhythms in swimming by the last larval stage (megalopa) of blue crabs Callinectes sapidus and fiddler crabs Uca spp. Previous field studies found that megalopae of both species were only abundant in the estuarine water column on nocturnal rising tides. Megalopae were collected from the Newport River Estuary, North Carolina (34°41′N; 76°40′W) during August–September 1992 and swimming activity was recorded for 4.5 to 7 d under constant conditions with a video system. Rhythms exhibited by both genera in the laboratory were not identical to those recorded in the field. Uca spp. displayed a circatidal rhythm, with maximum swimming occurring near the time of high tide in the field. Rhythm amplitude increased when crushed oyster shells were present, which suggested that megalopae bury or cling to the substrate during quiescent periods. In contrast, C. sapidus had a circadian rhythm in which maximum swimming coincided with the day phase in the field. In most trials, the activity of blue crab megalopae was unrelated to the expected tidal cycle. It was concluded that a tidal rhythm in swimming was the behavioral basis of flood-tide transport for fiddler crab larvae. The endogenous rhythm in blue crabs does not participate in transport, which probably results from behavioral responses to environmental cues associated with flood tide.

Megalopal Spatial Distribution and Stock Separation in Dungeness Crab (Cancer magister)

During the day, Dungeness crab (Cancer magister) megalopae from off the outer coasts of Vancouver Island and Washington are aggregated at about 25 m whereas those from the Strait of Georgia are at about 160 m. At night, both populations of megalopae seem to be mostly in the top metre of water. Juan de Fuca Strait, which connects the Strait of Georgia to the Pacific Ocean, typically has an estuarine circulation, with outflow in the top 50–100 m and deeper inflow. Because the daylight to dark ratio when megalopae are present is about 3:1, the Strait of Georgia and outer-coast megalopae are mostly retained within their own systems by currents at their daytime depths. Occasional intrusions of outer-coast megalopae into Juan de Fuca Strait may occur when estuarine flow in the Strait temporarily breaks down following sustained, strong, southwesterly winds; such intrusions are typically restricted to the south and head of Juan de Fuca Strait, and even extensive ones do not carry megalopae far into the Strait of Georgia. The daily movement of larval crab to cold (<10 °C), deep water in the Strait of Georgia may explain, at least partially, the delay in seasonal timing of settlement and their smaller physical size at settlement compared with outer-coast megalopae.

Swimming Velocities and Behavior of Blue Crab (Callinectes sapidus Rathbun) Megalopae in Still and Flowing Water

Habitat selection capabilities of the recruiting larval stages of marine invertebrates are limited, in part, by their ability to maneuver in flowing water. Distributional and experimental evidence suggest that blue crab (Callinectes sapidus) megalopae may preferentially settle into vegetated habitats. However, the behavior and swimming capabilities of megalopae in flowing water have not previously been investigated. Laboratory experiments were conducted in a small, recirculating seawater flume to determine the swimming response of megalopae to varying flow velocities. Nighttime trials were conducted at six flow velocities: 0, 1.9, 3.6, 4.8, 6.3, and 9.3 cm s−1. Behavior and swimming velocities of field-collected C. sapidus megalopae were video recorded. Megalopae exhibited negative phototaxis and were found in the water column at all flows in the dark. The maximum sustained swimming speed observed was 12.6 cm s−1 and the mean swimming speed in still water was 5.0 cm s−1, with short bursts in excess of 20 cm s−1. Megalopae frequently oriented into the current and were capable of swimming upstream against the current at flow speeds <4.8 cm s−1; at greater velocities they were not able to do so. The results suggest that at low to moderate current velocities C. sapidus megalopae have the ability to actively move in search of settlement sites and to maintain their positions in desirable sites rather than relying strictly on passive movements by currents.

Relationship of habitat and spatial scale with physiological state and settlement of blue crab posttarvae in Chesapeake Bay

Physiological state (molt stage) of planktonic and benthic blue crab (Callinectes sapidus) postlarvae (megalopae) was quantified within and outside Chesapeake Bay at various spatial scales. Physiological state of planktonic megalopae advanced significantly from the continental shelf off the Chesapeake Bay mouth, through the bay mainstem, and into upriver stations in the York River, a tributary of Chesapeake Bay. These results imply that settlement and metamorphosis of blue crab megalopae is dependent on location relative to the shelf. The physiological evidence supports the export-reinvasion theory of blue crab recruitment and is inconsistent with a larval retent~on hypothesis. In the tributary, benth~c megalopae were significantly more advanced in molt stage state than planktonic rnegalopae. Temporal variation in molt stage was observed over days and months. In addition, time to metamorphosis was significantly and positively correlated with molt stage. Advancement in physiological state of megalopae during reinvasion of the estuary may serve as an indicator of likelihood of settlement.

Competency, settling behavior, and postsettlement aggregation by porcelain crab megalopae (Anomura : Porcellanidae)

Gregarious settlement by megalopae of the porcelain crabs Petrolisthes cinctipes (Randall) and P. eriomerus Stimpson was investigated to determine possible means of attraction and postsettlement benefits. Megalopae of P. eriomerus were attracted to conspecific adults confined to chambers that prevented tactile or visual contact, suggesting that postlarvae respond to a waterborne cue. There was also clear evidence of an extended period of competency for settlement, since newly molted megalopae of P. cinctipes held with conspecific adults settled within 2–4 days while those without adults delayed settlement for 2–3 wk. Loss of swimming ability at settlement is followed by degeneration of the pleopods and color changes. Following settlement juvenile instars appear to gain protection by continuing to hide between and beneath conspecific adults; this behavior significantly reduced predation in a laboratory experiment.

Predation on postlarvae of the blue crab Callinectes sapidus Rathbun by sand shrimp Crangon septemspinosa Say and grass shrimp Palaemonetes pugio Holthuis

The megalopal stage ( = postlarva) of the blue crab Callinectes sapidus Rathbun enters Chesapeake Bay after larval development in coastal waters and settles in shallow areas of lower Chesapeake Bay. We examined the effects of two potential predators, two megalopal densities and two levels of habitat complexity on mortality of blue crab megalopae in laboratory experiments. Sand shrimp Crangon septemspinosa Say consumed > 99% of megalopae in each of two 40-h trials. Grass shrimp Palaemonetes pugio Holthuis consumed 37% of megalopae in the first trial, but only 5% in the second trial. No effect of habitat (sand or sand with artificial seagrass) or prey density on predation rates for either predator species was found. We then tested the hypothesis that megalopae undergoing metamorphosis to the first juvenile instar suffer higher mortality to grass shrimp predation than megalopae not in the process of metamorphosis. This hypothesis was rejected because predation rates did not differ between treatments with 0 or 95% metamorphosis. Higher predation rates (77.4%) by grass shrimp in the second set of experiments were associated with warmer water temperature, though temperature was not included formally as a factor in the design. These shrimps are seasonally abundant in Chesapeake Bay grass beds and capable predators of blue crab megalopae in the laboratory; field studies will be necessary to determine their impact on blue crab recruitment success.

Flexibility in timing of the metamorphic molt by fiddler crab megalopae Uca pugilator

The larvae of many invertebrate taxa can delay metamorphosis for considerable periods of time in the absence of specific stimuli for settlement. This study tested the hypothesis that the duration of the megalopal stage of fiddler crabs can be adjusted by several days depending on the availability of some habitat components. The hypothesis that megalopae can delay the metamorphic molt was tested by comparing the proportion of individuals molting to the first crab instar in aquaria containing only filtered seawater to that in aquaria with mud, sand, and/or an adult crab, near the predicted time of molting. The hypothesis that molting can be advanced was tested by comparing the proportion of sibling larvae molting at intervals throughout the megalopal stage in aquaria containing only filtered seawater or seawater plus an adult crab, mud, and sand. Megalopae delayed molting unless 2 stimuli were present: an adult crab and sediment, either mud or sand. The metamorphic molt was advanced in the presence of both these cues, although contact with the sediment by the adult crab appears necessary for maximal effect. Flexibility in timing of molting and the requirement for more than one cue can increase the chance that megalopae settle and molt to the first crab instar in appropriate adult habitats.

Planktonic availability, molt stage and settlement of blue crab postlarvae

We quantified (1) spatio-temporal variation in molt stage (developmental state) of blue crab Callinectes sapidus megalopae (postlarvae), and (2) the relationship between planktonic availability, molt stage and settlement of megalopae during peak settlement in Chesapeake Bay, USA. Settlement was significantly correlated with the planktonic availability of megalopae. Developmental state of megalopae also appeared influential in settlement because (1) blue crab megalopae displayed quantifiable changes in molt stage; (2) molt stage of megalopae varied on a temporal scale of days to 1 mo or more, and a spatial scale of kilometers, apparently reflecting the physiological progression through the molt cycle by megalopae pulsing through settlement habitats; and (3) molt stage of megalopae advanced in collections from the plankton, on artificial settlement habitats, and in the benthos, indicating the approach to settlement, metamorphosis and a benthic existence. In addition, settlement was not necessarily obligatory untd sometime in late premolt, as many megalopae collected in the plankton were in late premolt. Hence, megalopae have the time in which to exercise some degree of habitat selection. Our findings show that settlement of a benthic invertebrate is regulated partly by the planktonic availability of postlarvae, and that these postlarvae progress through a predictable series of changes in developmental state as they invade settlement habitats. Thus, a quantitative measure of postlarval developmental state appears useful in relating planktonic availability of postlarvae to natural settlement patterns. Similar alterations in developmental state occur in the larval and postlarval stages of marine fish and a wide range of invertebrate taxa, indicating the potentially broad applicability of quantitative measures of developmental state in studies of settlement.

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.

Marine food and diet overlap of co-occurring Arctic charr Salvelinus alpinus (L.), brown trout Salmo trutta L. and Atlantic salmon S. salar L. off Senja, N. Norway

Stomach contents analyses and other biological information of Arctic charr (Savelinus alpinus (L.)), brown trout (Salmo trutta L.) and small Atlantic salmon (S. salar L.) caught 1982–85 close to the Aelv estuary (69°N) on the island of Senja, N. Norway are presented, and extracts of a 1975–85 fishing log given. this appears to be the first case study of the feeding habits of all three European anadromous salmonids in marine sympatry, and also one of very few reports on the marine food of the Arctic charr from Europe. The general feeding habits of the charr were similar to that found in N. Canada. Pelagic fish (herring, sand-eel) seem to be preferred. Plankton (crab megalopae, krill) and hyperbenthos (amphipods, mysids) are also taken, especially when suitable fish are scarce. In 1985 high herring densities provided superabundant food, and diet overlap between charr, trout and salmon was high. Salmonid nursery rivers are abundant in N. Norway and during summer the three species coexist in a near-shore, surface-oriented pelagic guild of fishes. The salmon seems to be a relatively specialized piscivore, while the trout takes a wider range of fish and also invertebrate prey. The charr probably is the most euryphagous of the three, being able to exploit the more marginal parts of the prey resources of their common habitat.

The diet of jackass penguins Spheniscus demersus in Algoa Bay, South Africa, and its bearing on population declines elsewhere

Jackass penguins Spheniscus demersus were sampled monthly over a two-year period with a stomach sampling device at St Croix Island in Algoa Bay on the east coast of South Africa. Their diet comprised 19 species of fish, one cephalopod species and a crab megalopa larva. Pelagic shoaling fish were the dominant prey items, specifically anchovy, pilchard and round herring, in order of importance. Thereis no pelagic fishery on the east coast, but the dominant prey species of jackass penguins are the target species of purse seine trawlers on the west coast. Findings in this study tend to support the hypothesis that over-exploitation of pelagic fish stocks could have been responsible for the continuing decline of jackass penguins on islands off the west coast of Southern Africa.

Surface swarms of swimming crab megalopae at Curaçao (Decapoda, Brachyura) : Rice, A.L. and I. Kristensen, 1982. Crustaceana, 42(3):233–240

Surface swarms of swimming crab megalopae at Curaçao (Decapoda, Brachyura) : Rice, A.L. and I. Kristensen, 1982. Crustaceana, 42(3):233–240