Seasonal Recruitment Research Articles

Connectivity between juvenile and adult fish populations: do adults remain near their recruitment estuaries?

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 240:215-223 (2002) - doi:10.3354/meps240215 Connectivity between juvenile and adult fish populations: do adults remain near their recruitment estuaries? Bronwyn M. Gillanders* Department of Environmental Biology, University of Adelaide, Adelaide, South Australia 5005, Australia *E-mail: bronwyn.gillanders@adelaide.edu.au ABSTRACT: Juvenile snapper Pagrus auratus (Sparidae) were collected from 15 estuaries during the recruitment season of 1998-99 and their otoliths were analysed to determine concentrations of chemical elements. Differences in elemental composition of juveniles were found among estuaries or groups of estuaries suggesting that the nursery or recruitment estuary of adult fish could be determined by analysing the juvenile region of adult otoliths. Adult fish from the commercial fishery in the vicinity of Sydney, New South Wales, Australia were then collected, their ages estimated and fish with birth years during 1998-99 selected for microchemical analysis to determine their recruitment estuary. Maximum likelihood analyses were used to determine the proportion of juvenile fish and adult fish from different regions (Wallis Lake, Sydney estuaries, Eden, and other estuaries). For juvenile fish, the actual composition ranged from 7 to 53% depending on the estuary or group of estuaries and the estimate of proportion of juveniles from the different estuaries ranged from 7.24 to 48.21% suggesting an error rate of <1 to 4.79%. Most (89%) adult fish caught as part of the snapper fishery in the Sydney region originated from local estuaries, although about 9% of fish had Eden as their recruitment estuary and 2% had come from the remaining estuaries excluding Wallis Lake, which contributed no fish. These results suggest that adults on reefs outside estuaries in the Sydney region have come from the estuaries closest to them with little transfer from other estuaries. For other regions, 2 scenarios are possible, namely, adults originated from estuaries close by with little transfer from other estuaries, or alternatively estuaries in the vicinity of Sydney supply the majority of adult fish along the coast. The latter situation is considered unlikely. Results show a link between juvenile and adult populations and suggest that populations of snapper may be self-sustaining or relatively closed. KEY WORDS: Estuary · Coastal population · Fish · Otolith chemistry · Trace elements Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 240. Online publication date: September 12, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

SEASONALITY, PARASITE DIVERSITY, AND LOCAL EXTINCTIONS INPLASMODIUM FALCIPARUMMALARIA

We incorporate stochastic, density-dependent seasonal recruitment in adult Anopheles mosquito populations in a discrete-event model of Plasmodium falciparum malaria transmission and find the probabilities of parasite extinction higher than with perennial transmission. Seasonal fluctuations in vector populations act to synchronize the dynamics of infection and immunity in host populations, leading to fluctuations in parasite prevalence greater than expected solely on the basis of high- and low-season vector densities. This synchronization also biases frequencies of infection with multiple parasite phenotypes or genotypes.

Seasonal dynamics and recruitment strategies of the invasive seaweed Grateloupia doryphora (Halymeniaceae, Rhodophyta) in Narragansett Bay and Rhode Island Sound, Rhode Island, USA

Although there is considerable interest in the introduction and spread of invasive species, there has been little research documenting growth patterns of a specific organism from the time of its first arrival. This paper addresses the seasonality and recruitment of Grateloupia doryphora, a large red algal species that arrived in Narragansett Bay, Rhode Island, USA, in 1994. From 1997 to 1999 inclusive, this species exhibited a distinct seasonal pattern, with the lowest percent cover occurring in May–June and the highest percent cover in October. This timing of low and high percent cover was observed concurrently at three stations over a two-year period. Percent cover was a function of blade size and numbers. New blades appeared year-round. In addition to percent cover, blade colour changed seasonally from light red (June–August) to dark burgundy (August–January) to yellow red (February–May). Strategies for recruitment of G. doryphora were observed in the field, on both natural and artificial substrata (fixed plastic plates coated with sand), and in culture. Recruitment on sand-coated plates in Narragansett Bay and Rhode Island Sound resulted from spore settlement and subsequent crust and blade development. On a single plate, hundreds of juvenile upright blades appeared in May–June 1999, and the largest blades grew to 25 cm long by August. In addition to recruitment on plates, crusts appeared on bare rock, and new blades arose from crusts and from proliferations of older blades. Laboratory cultures clarified the developmental sequence from spore to crust to blade and/or upright filaments to new crusts and new blades. The crustose form may provide a strategy for survival during unfavourable environmental conditions, enabling G. doryphora to establish residency in Rhode Island waters. The effect of this invasive species on other organisms has yet to be determined.

Patterns in Species Composition of Fish and Selected Invertebrate Assemblages in Estuarine Subregions near Ponce de Leon Inlet, Florida

The Florida Fish and Wildlife Conservation Commission's Fisheries-Independent Monitoring program monitored the species composition and relative abundance of fishes and selected invertebrates in the estuarine waters near Ponce de Leon Inlet, Florida, from January 1993 through to December 1996. Sampling sites were located in three distinct physiographic and biotic estuarine subregions: Mosquito Lagoon (ML; 11 stations), Ponce de Leon Inlet (PI; 8 stations), and Tomoka Basin/River (TR; 8 stations). This series of subregions associated with Ponce de Leon Inlet is a species-rich estuarine system that is numerically dominated by a few taxa. A total of 1080477 animals representing 160 species and 56 families were recorded.Anchoa spp. (29·9%) and Menidia spp. (12·6%) were the numerically dominant taxa. Sciaenidae and Gobiidae were the most speciose families, with 12 species each (14·0% and 1·7% of the total animals collected, respectively), followed by the bothids 10 species (0·6% of total animals). Detrended correspondence analysis showed that the subregions could be discriminated by their respective species composition and relative abundance. Spatial differences in species composition were principally attributed to differences in the magnitude of seasonal recruitment events and to habitat characteristics associated with the presence of seagrass, inlet dynamics, or the influence of freshwater discharge. The presence of freshwater taxa (centrarchids) and the seasonal recruitment of juvenile Penaeidae,Micropogonias undulatus , Stellifer lanceolatus and Leiostomus xanthurus, characterize the TR area, whereas the presence of species associated with higher salinities (Opisthonema oglinum and Harengula jaguana) characterize PI and with seagrass (Lucania parva and Lagodon rhomboides) characterize ML.

A new look at computation of the complexity index in mangroves: do disturbed forests have clues to analyze canopy height patchiness?

This paper proposes some guidelines to compute complexity index in those mangroves where either seasonal or strong disturbances have occurred. We surveyed 31 mangrove localities in Buenaventura Bay, Central Pacific Coast of Colombia, where structural parameters were measured within a 0.1 ha plot. Also, most likely disturbances were noted for each plot. Complexity index was calculated in its classical form using the arithmetic mean of the three tallest trees(maximal mean) and alternatively using: a) the total mean: the height average of all trees recorded in each plot; and b) the mode: the most frequent tree height class (10 cm intervals) within each plot. Afterwards, we compared the three computations and discussed there liability of each one according to the current state by plot. In addition, all structural parameters were sorted in two diameter at breast height (dbh) cohorts (2.5–10 cm and ≥ 10 cm) to figure out which contributes more to the forest structure. We conclude the following: (a) The mean of the three tallest trees is not a good estimator of forest development when seasonal or strong disturbances occur since complexity index based in it always overestimates forest structure. (b) Seasonal disturbances and recruitment produce mosaic forests. The best estimator of this condition is the mean height which encompasses both central tendency and variability. (c) The modal height is also helpful to establish the dominant cohort when forests show two or more storied-canopies, or intermediate cohorts are missing. It also applies when a new stock of recruits is entering in a mature forest (the modal or maximal heights can be used interchangeably in this case). (d) Maximal height is the best estimator for uniformly developed forests with closed canopies and/or a single dominant tall-cohort. (e) If one is not confident about which height type to include to compute the complexity index, we recommend to sort out structure data by dbh-cohorts and calculate indices for both of them. This will show which cohort is contributing most to forest complexity. Finally, we suggest to exclude non-mangrove species from the complexity index computation since they mostly do not contribute significantly to forest basal area.

Seasonal abundance and recruitment in an estuarine population of mud crabs, Scylla paramamosain, in the Mekong Delta, Vietnam

Artisanal fisheries for mud crabs are an important source of income in coastal communities of the Mekong Delta. However, populations are subject to pressure from increasing fishing effort to provide seed crabs for pond culture, as well as diminishing mangrove habitats. In the present study, Scylla paramamosain was found to be the dominant mud crab species within an estuarine mangal system, representing over 96.8% of mud crab fishery landings between February 1998 and March 1999. S. olivacea was also present in relatively low abundance (3.2% of landings). Analysis of catch-per-unit-effort (CPUE) data collected throughout 1997 and 1998, for an intertidal hand-fishery, shows that mud crabs were present in a fairly narrow range of mean abundance throughout the year, despite protracted periods of freshwater conditions during the rainy season. Recruitment of juvenile S. paramamosain (3–4 cm carapace width) was continuous through 1998, with a significant peak at the beginning of the dry season. No juvenile S. olivacea (< 5 cm carapace width) were found during the study. The mangal appears to provide a nursery habitat for S. paramamosain, with predominantly juveniles and sub-adults present, while a higher proportion of adults were recorded in the sub-tidal component of the population. Mature females were present throughout the year, with a peak in September–October 1997. The CPUE analysis provides a baseline of seasonal variation in crab abundance that can be used to monitor the effectiveness of resource management and potential impacts of future changes in habitat.

DISTURBANCE, HABITAT STRUCTURE, AND THE DYNAMICS OF A CORAL-REEF FISH COMMUNITY

Coral reef fishes occupy habitats that are patchy and subject to frequent natural disturbances. Although different types of disturbance are likely to generate different community responses, the relationship between different disturbance agents and their effects on reef fish communities has not been examined experimentally. We studied a set of natural patch reefs, dominated by a diverse array of soft and hard coral cover, at Lizard Island on the Great Barrier Reef (northeastern Australia). The fish assemblages on the reefs were sampled over 4 mo to establish baseline values and then experimentally disturbed. Two types of disturbance were carried out in a factorial combination: pulsed mortality by removing all fish from reefs and pulsed habitat disturbance. Habitat disturbance was applied at two levels: Level 1 consisted only of damaging all live hard corals with a hammer; Level 2 consisted of damaging all live hard corals, and in addition, using a hammer to reduce the height and complexity of the reef matrix. We then monitored the experiment for a further 19 mo, including two recruitment seasons. Unmanipulated control assemblages persisted through time, and despite large changes in total abundance, species composition remained consistent relative to disturbed treatments. Assemblages disturbed by fish removal were resilient, with recolonization from both immigration and larval settlement effectively removing differences between removal treatments and controls 3 mo after manipulation. Habitat disturbance alone generated differences between experimental and control assemblages, which persisted for the duration of the experiment. The more extreme level of habitat disturbance generated more extreme changes in fish assemblages when no pulsed mortality occurred. Habitat disturbance in combination with pulsed mortality generated similar community responses as the habitat disturbance treatment alone. However, fish removal had the effect of eliminating the difference between fish assemblages on reefs subjected to different levels of habitat disturbance. Community response to habitat disturbance was driven by species-specific patterns of reduced abundance of species associated with live coral in combination with increased numbers of those associated with rubble. Declines in the abundance of coral associates on damaged reefs were abrupt, with no recovery observed for the duration of the experiment. In contrast, increases in the abundance of rubble associates were more ephemeral, in that initial high levels of recruitment and immigration were followed by a high rate of loss. Habitat disturbance also generated reefs that typically supported lower fish abundance, fewer species, and increased evenness relative to controls. Our results support a model of patch-reef fish assemblages organized by a combination of deterministic factors (such as habitat structure) and stochastic processes (such as recruitment). These disparate mechanisms operate in concert to generate reasonably consistent patterns of community structure. Habitat structure appears to mediate much of the apparent determinism and is likely to operate both as a reflection of species-specific habitat preferences and by modifying interactions among fish species. Consequently, disturbance plays a substantial role in structuring communities of coral-reef fishes by modifying both spatial and temporal heterogeneity.

Variation in barnacle recruitment over small scales: larval predation by adults and maintenance of community pattern.

Over small spatial scales, variation in the density of settlers of benthic sessile species is the result of interactions among larval behavior, local hydrodynamic conditions, and the physical, chemical and biological characteristics of the benthic habitat. It has been shown repeatedly that adult benthic filter-feeders can consume larvae of their own and other species, but their effects on the distribution and abundance of recruits have rarely been demonstrated under natural conditions in the field, particularly on hard substrata. Here we experimentally quantified the effect of the large intertidal barnacle, Semibalanus cariosus (Pallas), on the density of recruits of three common barnacle species. The experiments were conducted at the peak of the barnacle recruitment season over three successive years, on the west coast of San Juan Island, Washington. A persistent and well documented community pattern in the mid intertidal zone of the study site is a sparse bed of adult S. cariosus with bare rock spaces essentially devoid of small barnacles among the large individuals. Field experiments consisted of small areas from which either all adult S. cariosus were killed leaving the shells attached to the rock, or live adult barnacles were left intact. Our results showed that over small spatial scales of a few to tens of centimeters, the large barnacle S. cariosus can interfere and significantly reduce net settlement and recruitment of conspecific as well as other barnacle species. Between 65 and 100% reduction in settlement could be attributed to larval predation by adults, as implied by barnacle settlement patterns on different treatments and by the presence of nauplius larvae in cirri and stomach contents of S. cariosus. The negative effect on barnacle settlement was consistent between years of relatively low barnacle recruitment, which appears to be the most common situation at the study site, but it disappeared on a year of unusually high recruitment, when settling larvae seem to have swamped the filtration ability of adult S. cariosus. The different barnacle species displayed contrasting settlement patterns on bare rock and on the lateral shells of the large barnacles, which appear to be a result of differences in larval behavior. Comparisons against the relative availability of these substrata in the experimental plots suggested that larvae of different species sample the benthic microhabitat in very different ways.

Dominance of Cylindrospermopsis raciborskii (Nostocales, Cyanoprokaryota) in Queensland tropical and subtropical reservoirs: Implications for monitoring and management

AbstractSince October 1997, 47 reservoirs and weir pools across tropical and subtropical Queensland have been regularly monitored for the occurrence of planktic cyanoprokaryotes. Cylindrospermopsis raciborskii (Woloszynska) Seenaya &amp; Subba Raju (Nostocales, Cyanoprokaryota) was found in 70% of the storages, with one storage displaying year‐round dominance, 50% of the reservoirs seasonally dominated and a seasonal presence in 46% of the weir pools. Maxima for the majority of storages occurred from late summer through to early autumn. The precise timing of onset of seasonal maxima varied considerably between storages and regions. Temperature and stratification patterns influenced seasonal recruitment with C. raciborskii reaching seasonal maxima in southern storages generally later than the northern storages. Overall peak seasonal abundance occurred in deep strongly stratified storages. The majority of storages experiencing concentrations &gt; 15 000 cells mL–1 tested positive to the presence of the alkaloid cytotoxin, cylindrospermopsin. Median cylindrospermopsin concentration across the 14 reservoirs in which toxin was recorded was 3.4 μg L–1. The highest toxin concentrations were generally associated with storages in which C. raciborskii had been established for a considerable period of time, or occurred after the peak summer population maxima. Toxin concentrations of 1 μg L–1 were generally associated with cell concentrations of approximately 20 000 cells mL–1, hence this cell concentration threshold was adopted as a health trigger level at which to begin monitoring for toxicity. The morphology of this species was highly variable and included straight, coiled and sigmoid‐shaped trichomes. Populations were routinely recorded as mixtures of all three morphotypes or proceeded as transitions from one morphotype to another throughout the year. The dominance of C. raciborskii appears to be favoured by a set of environmental and hydrological factors that include long water residence time, high pH, high temperature, high incident irradiation and a thermally stratified water column. The lack of visual monitoring cues, such as scum formation, variation in colour of the water body, rapid germination of large numbers of cells, highly variable morphology, relative toxicity and persistence of this species year round in many areas, continues to make this species a primary focus of water managers.

Breeding Phenology of the Lowland Leopard Frog (Rana yavapaiensis): Implications for Conservation and Ecology

Breeding Phenology of the Lowland Leopard Frog (Rana yavapaiensis): Implications for Conservation and Ecology

Fish assemblage on natural versus vertical artificial reefs: the rehabilitation perspective

Artificial reefs have been suggested as a potential tool for the restoration of marine habitats. In the present study, the fish assemblage established around the oil jetties of Eilat (northern Red Sea, Israel) was compared to those found in three adjacent natural reef habitats: two in a nature reserve (one shallow and one deep) and a third deep site located near the city. Both species richness and fish abundance were found to be significantly higher around the vertical structures of the jetty's pillars than at all three natural sites, with the lowest values at the site closest to the city. The higher species richness at the jetties may be explained by (1) the vertical relief and high complexity of the jetty which offers a variety of niches for both shallow and deep coral reef species, and (2) by the reduction in available niches at the natural sites as a result of coral destruction due to anthropogenic activity. The pronounced difference in fish abundance is attributed mainly to the high seasonal recruitment at the jetty which was much lower at the natural sites. We therefore suggest that vertical structures are more attractive to fish settlement and recruitment than moderately sloped bottoms such as those found at the fringing reefs of Eilat. High similarity (51 to 56%) was found between fish assemblages at the natural sites while relatively low similarity (27 to 37%) was found between the jetty and the natural reefs. The jetty's complex vertical artificial structures can serve as a model for future construction of artificial reefs designed to restore the fish community in areas where the natural reefs have been damaged. It should be taken into account, however, that these do not necessarily mimic the natural environment but may rather establish a community of their own, which is influenced by the spatial orientation and complexity of the structure.

Perspective of the pink shrimp (Farfantepenaeus duorarum burkenroad) fishery of Campeche bank, Mexico

The pink shrimp, Farfantepenaeus duorarum, fishery of Campeche Bank, Mexico, was analyzed in order to characterize it and to test the effect of different fishing intensities upon the stock. Catch data consisted of the number of shrimp per commercial class for the 1986–1993 period. A virtual population analysis structured by age was applied to determine seasonal recruitment and fishing mortality patterns with respect to age, as well as the stock-recruitment relationship. With these results, an age-structured deterministic simulation model was developed, which diagnosed the fishery and defined the effects of different exploitation scenarios by considering changes in fishing mortality. Results show that the decreasing catch tendency through the period of analysis is related to recruitment failures. The analysis indicated that a way to mitigate the low recruitment problem is to control and reduce fishing mortality, but stock density will continue to decrease if the current fishing intensity is maintained.

Temporal variation in early mortality of an intertidal barnacle

During the period following settlement to the substratum and preceding metamorphosis, dispersive larval stages of marine invertebrates must undergo a physiologi- cally demanding metamorphosis, after which juveniles are exposed to new environmental conditions. Given that the intensity of biological and physical disturbance and larval physiological quality can vary over time, it would be expected that survival and growth would also vary among daily cohorts of settlers arriving to the benthic habitat. The objectives of this field study were to determine the extent to which daily cohorts of settlers of the barnacle Semibalanus balanoides differ in early post-metamorphic (i.e., juvenile) growth and survival, and to compare mortality of early juveniles and newly attached, pre-metamorphic individuals (i.e., cyprids). Mean mortality of all individuals 30 d after settlement differed significantly among the 5 daily cohorts examined, ranging from 29.9 to 70.2%. Mean mortality also differed significantly among the 5 daily cohorts for cyprids (range 15.3 to 43%) and juveniles (range 7.1 to 30.8%), indicating that mortality of cyprids is generally at least as great as early juvenile mor- tality. Mean juvenile growth differed significantly among the 4 cohorts examined, with cohorts arriving during the begin- ning of the recruitment season exhibiting the fastest growth. These results support previous reports of significant variation in mortality and growth among daily cohorts of barnacles. It is proposed that temporal variation in cyprid mortality may weaken the strength of the relationship between larval abun- dance and recruitment for populations of marine inverte- brates.

Temporal variation of recruits as a basis of ephemeral genetic heterogeneity in the western rock lobster Panulirus cygnus

Allozymes were examined in quantitative lunar monthly collections of larval recruits of the western rock lobster Panulirus cygnus George over three recruitment seasons at two sites nearly 350 km apart in Western Australia. At Alkimos, the southern site, recruitment occurs in a relatively narrow peak early in the spring, whereas at the northern Houtman Abrolhos Islands, recruitment extends into the summer months. In the 1995/1996 recruitment season, the frequency of the GPI*100 allele increased from early to late in the season, but the frequencies were indistinguishable at the two sites in each monthly collection. The combination of this temporal variation in allelic frequencies with the contrasting patterns of recruitment at the Abrolhos Islands and Alkimos resulted in genetically different cohorts at the two sites. This pattern was ephemeral, as it was not repeated in the subsequent two years. Thus, ephemeral genetic patchiness in P. cygnus can be generated by the locally-specific genetic mix of recruits obtained from a common larval pool. This mechanism is the probable explanation of previously observed temporal and possible spatial genetic variation in adult P. cygnus, and highlights the importance of studying recruitment in order to understand the genetic structure of marine species.

Seasonal growth and recruitment of Himanthalia elongata (Fucales, Phaeophycota) in different habitats on the Irish west coast

Seasonal growth and recruitment of Himanthalia elongata (Fucales, Phaeophycota) in different habitats on the Irish west coast

Seasonal growth and recruitment of Himanthalia elongata Fucales, Phaeophycota) in different habitats on the Irish west coast

Vegetative and reproductive growth of individually marked plants of the brown alga Himanthalia elongata was monitored over 2·5 years at two sites with different wave exposures on the Irish west coast. Macro-recruits were first visible to the unaided eye in February/March. About 65% of all buttons produced receptacles during autumn of the same year, whereas others remained sterile. Growth of receptacles started between late August and late November, when vegetative plants were 9–14 months old, but no further plants became reproductive after December. At both sites, length increase of receptacles was greatest between February and April, but only longitudinal growth occurred and no more new dichotomies were produced. At the more wave-exposed site, growth was fastest in plants growing in red algal turfs and slowest in plants on open rock. Growth rates were maximal when seawater temperatures were 10–12 °C in early spring but decreased from May onwards with the maturation of receptacles aged 7–9 months in June. Release of viable gametes started in June and continued throughout the autumn and winter, until all plants disappeared from the population in February. Plant density was highest in March after the appearance of macro-recruits in the population, and declined during summer when the number of buttons decreased. Percentage plant cover was highest in June immediately before the onset of gamete release and this was followed by the decay and disintegration of receptacles. Since both survival and growth of young buttons was highest in red algal turfs and lowest on exposed rocks, the distribution of H. elongata on semi-exposed shores in Ireland may be limited by the availability of suitable micro-habitats for recruitment, and reduced by the early loss of fertile, adult plants from exposed sites.

Ecology of the polychaete Nereis diversicolor in the Canal de Mira (Ria de Aveiro, Portugal): Population dynamics, production and oogenic cycle

The population of Nereis diversicolor O.F. Müller was studied in 1993 and 1994 at three intertidal areas along the estuarine gradient in the Canal de Mira (Ria de Aveiro). The population dynamics, secondary production and growth were followed for the 2-year period, whereas in the second year, a study on the oogenic cycle was also carried out. For the population dynamics and production studies, core samples were collected monthly at each site, and for the study of the oogenic cycle, medium size and large individuals were sampled from the sediment. Mean annual densities were higher (573–718 ind.m–2) at a station located in the middle zone of the channel, followed by the outer (190–275 ind.m–2) and the innermost (43–94 ind.m–2) stations. A cohort analysis by `Bhatta' software enabled the separation of the cohorts present at each date, and two cohorts were followed from recruitment to extinction. The secondary production ranged from 15.9 to 74.2 g ash free dry weight.m–2.year–1 and the P/Bratio from 4.4 to 7.9. The average body growth rate varied between 0.16 and 0.20 mm.d–1 for worms longer than 2 cm. Two recruitment seasons were detected each year. Two spawning periods were also observed, one in the spring and the other in early autumn.

Population structure of six sympatric species of Trapezia associated with the hermatypic coral Pociliopora damicornis with a hypothesis of mechanisms promoting their coexistence

Seasonal changes in species composition and size frequency distributions of six sympatric coexisting species of Trapezia associated with the hermatypic coral Pociliopora damicornis were investigated to analyze their population structure, mechanisms promoting coexistence, and the organizing processes of the symbiont community. All Trapezia species showed a long recruitment season, but they could be divided into two groups according to variations in their size frequency distribution pattern throughout the year: Young T. ferruginea and T. guttata occurred all year round, while young T. cymodoce, T. areolata, T. digitalis, and T. sp. were absent part of the time (a few months). Recruitment of T. cymodoce and T. areolata occurred from April to August and from February to March, respectively. Young T. digitalis were abundant only in July. Even among larger colonies, those harboring only one or two Trapezia species were rather common. This suggests that agonistic interactions occur among these crab species. Although it is also true that host colonies harboring several species are sometimes found. Since no conspicuous hierarchy in inter-specific agonistic interactions has been demonstrated (Tsuchiya and Yonaha, 1992), new crab recruits of any species can invade colonies lacking strong competitors.Although these crab species require very similar resources, i. e. pocilloporid corals as their habitat and mucus produced by host corals as food, their coexistence on larger single colonies is promoted by several mechanisms: 1) different recruitment seasons, 2) weak hierarchy of agonistic interactions among the six species, and 3) microhabitat segregation between adults and young on a colony.

The Epibenthos of the Backwaters of a Tropical Mangrove Creek (Tudor Creek, Mombasa, Kenya)

The epibenthos of the backwaters of Tudor creek (Mombasa, Kenya), a mangrove-bordered seasonal estuary, was sampled fortnightly with a beam trawl from May 1995 to April 1996. A total of 6396 specimens was collected in 96 tows. Almost 63% of these specimens belonged to 72 species of teleost fish (in 37 families). Penaeid (4 species) and caridean shrimp constituted 27% and 8% of the total catch respectively, while stomatopods and squids were quite rare. At family level, Penaeidae contributed most to the total catch. Gobiidae (15%), Lutjanidae (9%), Plotosidae (9%), Acropomatidae (6%), Gerreidae (5%), and Synodontidae (4%) were the most important fish families. Gobiidae was the most diverse family with 8 species, followed by Apogonidae and Lutjanidae with 5 species each, and Penaeidae with 4 species. Yongeichthys nebulosus (Gobiidae), Penaeus monodon, P. semisulcatus and Metapenaeus monoceros (Penaeidae), Plotosus lineatus (Plotosidae), Lutjanus fulviflamma (Lutjanidae), Acropoma japonicum (Acropomatidae), Gerres oyena (Gerreidae), and Saurida undosquamis (Synodontidae) were the dominant species (each contributed for more than 4% to the total catch). A TWINSPAN of the catch data identified three monsoon driven communities. Penaeus monodon, Lutjanus fulviflamma and Gerres oyena were the dominant species in the dry period (January to April), while P. semisulcatus and Caridea dominated the community in the long rainy season (May to August), and Acropoma japonicum and Plotosus lineatus were important during the short rains (September to December). Metapenaeus monoceros was abundant in all seasons and Yongeichthys nebulosus reached high densities in both short rainy and dry seasons. The short rains community had the highest numbers of species and densities, while those of the long rainy season were lowest. The three communities had very strong dominance of only a few species, conforming to the general observation that tropical estuarine or coastal ecosystems tend to be very diverse but with very low densities for the majority of species. The restricted size ranges for all individuals caught during this study may be due to two processes: selectivity of the gear and/or dynamic replacement of the populations through seasonal recruitment and fast size progression. All individuals collected during this study were juveniles. This fact and the observed monthly size distributions suggest that most species utilise the area as a nursery.

Population structure and dynamics of two scopimerine sand crabsScopimera crabricaudaAlcock 1900 andDotilla sulcata(Forskåll 1775) in an estuarine habitat in Oman

Monthly samples of the scopimerine sand crabs, Scopimera crabricauda Alcock 1900 and Dotilla sulcata (Forskall 1775), were collected from November 1992 to January 1994 from a marine inlet, Khawr Suwadi (23°45′N 57°47′E), in the Gulf of Oman. The crabs inhabited mobile sand banks and flats. S. crabricauda occurred at mean densities of 79 m−2, between low high water neap and mean low water neap, and D. sulcata occurred at mean densities of 39 m−2, below this and down to near mean low water spring. The largest individuals were males, but in general the adult crabs were small, ranging from 2.2–5.9 mm CW in S. crabricauda and 2.1–10.3 mm CW in D. sulcata. Juveniles of both species were present throughout the year, but seasonal recruitment varied between years and did not show a clear temporal relationship to the cycles of breeding as indicated by the percentage of ovigerous females in the population. In comparison with other members of the genera, female egg loads of both species were low. This can be accounte...