Settlement Pulses Research Articles

Condition, size, and winter duration affect winter survival probability of juvenile Atlantic cod Gadus morhua in a coastal subarctic ecosystem

Juvenile Atlantic cod Gadus morhua in coastal Newfoundland, Canada, settle in nearshore habitats in 3-6 pulsed events each year, from mid-summer to early winter, creating a broad size range of age-0 fish within each year-class, with potentially different survival trajectories entering their first winter. We hypothesized that early-arriving pulses, high autumn body condition, and low winter severity would contribute to high first-winter survival of age-0 cod. To test these hypotheses, we examined juvenile cod catch data, archived samples, and winter temperature records to determine the impact of winter duration, body condition, and settlement time on winter survival from 2001 to 2019. Settlement pulse, pre-winter condition, and winter duration had an interactive effect on survival. Fish condition did not vary across pulses within a given year-class, but we observed improved winter survival during long winters when body condition was high, with highest survival among the larger, early-arriving pulses of fish. Late-arriving pulses of fish were small-sized before winter onset, but these pulses unexpectedly survived better than pulses settling earlier in the season during short winters when body condition was high. However, all settlement pulses had a survival advantage when settlement occurred earlier on average (associated with warmer autumns), suggesting that increased growth and size within a pulse can also contribute to overwintering success. Our findings challenge some assumptions of overwintering ecology (e.g. ‘bigger is better’, low temperature stress) but illustrate the importance of cohort effects for overwintering survival in a sub-arctic marine ecosystem in a changing climate.

Allochthonous subsidies drive early recruitment of a subtropical foundation species

Terrestrial nutrients flushed to sea via coastal runoff may provide a trophic subsidy to filter‐feeders, which might be particularly relevant in oligotrophic waters. In this study, we investigated how allochthonous subsidies might modulate the settlement dynamics of the mytilid Brachidontes solisianus, a space monopoliser in subtropical Brazilian shores. We used fatty acid trophic markers to identify the dietary subsidies to nearshore waters during wet and dry periods, and integrated them with key ecological drivers (temperature, rainfall) and physiological factors (nutritional status, size at settlement) acting on early life stages. Like other mytilids, for B. solisianus primary settlement can be followed by resuspension and reattachment (secondary settlement). A higher share of primary settlers may signal localized recruitment, while a larger contribution of secondary settlers would indicate increased migration rates. Through redundancy analyses we found that rainfall was correlated with 1) nearshore fatty acid trophic markers of land origin and 2) the frequency of primary settlers during the wet period, when the share of terrestrial inputs was also highest (13% of nearshore suspended matter). Conditions appeared favorable to localized recruitment through trophic settlement triggers, as settlement pulses of pelagic drifters followed heavy rains. Further, lipid analyses of pediveligers revealed that during the dry period these competent larvae selectively incorporated the essential 22:6ω3 docosahexaenoic acid (DHA) into membrane phospholipids at the expense of reserve lipids, suggesting a dietary deficit. Secondary dispersal also prevailed, indicating that the chance for relocation through secondary migrations during dry periods is likely higher. Overall, our results suggest that coastal runoff leads to allochthonous trophic subsidies in the form of organic‐rich plumes which may strongly modulate the settlement rate and dispersal potential of B. solisianus. Subsidies from land to coastal waters may thus be critical for the stability of benthic communities in regions of low marine primary production.

Winter Flounder Navigate the Postsettlement Gauntlet with a Bet‐Hedging Strategy

AbstractWinter FlounderPseudopleuronectes americanus, a coastal flatfish species of historically economic and cultural importance, have declined throughout the past few decades within the southern New England and mid‐Atlantic region of the United States, reaching a low point of less than 9% of their historic biomass in 2009. Unusually high postsettlement mortality is thought to impose a critical recruitment bottleneck on the population, potentially stalling recovery of Winter Flounder populations despite management measures. Survival and growth during early life history play a key role in the recruitment dynamics of marine fishes. Spatiotemporal differences in these vital rates from young‐of‐the‐year (i.e., age‐0) Winter Flounder have been variously linked to environmental gradients, anthropogenic stressors, differences in the timing of settlement, and location. To better understand local declines in recruitment productivity, we assessed vital rates of age‐0 Winter Flounder in five different bays in Long Island, New York. A weekly or biweekly beam‐trawl survey targeting age‐0 Winter Flounder was implemented over five summers. We compared survey‐based estimates of age‐0 mortality and growth, finding significant differences between locations in growth but not mortality. A consistently high abundance of age‐0 Winter Flounder in Shinnecock Bay and Mattituck Creek was prolonged by a secondary settlement pulse later in the season. Hypothesizing that multiple settlement pulses are a bet‐hedging strategy against temporally varying environmental conditions, we compared mortality, growth, and occupied habitat conditions between settlement pulses (cohorts), finding differences in growth and habitat occupancy that varied across years.

Исследование возможностей оценки состояния структуры металла трубопроводов, находившихся в эксплуатации, ультразвуковым методом

Results of the first stage of ultrasonic researches of structure of metal of pipelines of the thermal power stations which are taking place in operation are submitted. Metal of marks of steel was exposed to tests: 15Х1М1ФИ and the St. 20 with a various operating time (time of operation). Samples of metal were exposed metallographic to the analysis, on the basis of given which (medium-sized grains , dispersions of distribution lnD — σD) theoretical calculation of parameters OUSE of pulses of a bottom signal in metal is carried out. The quantitative data on influence of thickness of a layer of a contact liquid between the probe and object of the control on working frequencies 5,0, 7,5 and 10 MHz for various medium-sized values of a grain of metal are received. Examples of settlement and experimental pulses and spectra of signals for frequency 5 MHz are given.

Risk-sensitive foraging does not explain condition-dependent choices in settling reef fish larvae.

The transition from the planktonic larval to the benthic adult stage in reef fishes is perilous, and involves decisions about habitat selection and group membership. These decisions are consequential because they are essentially permanent (many fish rarely leave their initial settlement habitat, at least for the first several days or weeks). In one common Caribbean reef fish, the bluehead wrasse (Thalassoma bifasciatum), settling larvae either join groups or remain solitary. Grouped fish have lower mortality rates but slightly slower growth rates, and fish that are smaller at the time of settlement are less likely to join groups. We hypothesized that the decision of smaller (i.e., lower condition) fish to remain solitary could be explained by risk-sensitive foraging: with less competition, solitary fish may have higher variance in foraging success, so that there is a chance of a high payoff (outweighing the increased mortality risk) despite the lack of a large difference in the average outcome. We tested this by comparing the mean, standard deviation, and maximum number of (a) prey items in stomach contents and (b) post-settlement growth rates (from otolith measurements) of solitary and grouped fish during two settlement pulses on St. Croix, US Virgin Islands. However, we did not find evidence to support our hypothesis, nor any evidence to support the earlier finding that fish in groups have lower average growth rates. Thus we must consider alternative explanations for the tendency of smaller fish to remain solitary, such as the likely costs of searching for and joining groups at the time of settlement. This study reinforces the value of larval and juvenile fish as a testbed for behavioral decisionmaking, because their recent growth history is recorded in their otoliths.

Timing of juvenile fish settlement at offshore oil platforms coincides with water mass advection into the Santa Barbara Channel, California

Recent pathways taken by pelagic juvenile fishes to offshore oil platforms were reconstructed from remotely sensed and in situ measurements of currents and hydrography. Juvenile fishes comprised 52.8% (16,952 of 23 species) of all individuals (32,080 juveniles and adults of 35 species) observed during scuba surveys conducted about twice per week at two platforms in the eastern Santa Barbara Channel from May to August 2004. Blacksmith, Chromis punctipinnis (Cooper, 1863), and rockfishes (genus Sebastes, at least 18 taxa) comprised 95.1% of the recruits. Almost all rockfishes recruited to the deepest part of the platforms surveyed (26 and 31 m), while most blacksmith recruited in shallower waters. The onset of the recruitment season for juvenile rockfishes (genus Sebastes, Scorpaenidae) coincided with the advection of a low salinity water mass into the channel from the Southern California Bight. Before arrival of this water mass, water at the platforms resembled upwelled, high salinity water around the Point Conception region at the western channel entrance. Settlement pulses of rockfishes and blacksmith were observed during advective events when salinity decreased in the upper 40 m and currents turned northwestward or intensified in that direction. Two abundant rockfish species [bocaccio, Sebastes paucispinis Ayres, 1854, and treefish, Sebastes serriceps (Jordan and Gilbert, 1880)] showed synchronous patterns of juvenile settlement between platforms separated by 7 km. Our findings indicate that currents from the bight, rather than from central California, supplied recruits to settlement habitat in the eastern channel and that the spatial scale of connectivity for some fish populations in this region is greater than the channel itself.

Blue crab (Callinectes sapidus) larval settlement in North Carolina: environmental forcing, recruit–stock relationships, and numerical modeling

AbstractEcosystem‐based fishery management requires an understanding of relationships between fisheries and environmental variability. The purposes of this study were to investigate (1) how environmental forcing drives variability in larval settlement of the blue crab Callinectes sapidus, (2) whether larval settlement data are useful for determining recruit–stock relationships, and (3) whether environmental data can be used to predict settlement. Megalopae settling on passive collectors in the Newport River estuary, NC, were collected nightly from September to November for 13 yr from 1993 to 2009. Observed settlement was compared with winds, tides, and predicted settlement (numerical model derived from environmental data) using cross‐correlation analysis. Weather events associated with settlement pulses were also identified. Recruit–stock relationships were developed by comparing observed and predicted settlement indices to lagged NC fishery data. Settlement was positively correlated with winds from the northeast, southeast and south in 12 of 13 yr and with maximum duration nighttime flood tides in all years. Settlement pulses occurred when high pressure systems formed over the southeast USA (58% of pulses) or hurricanes (25%), and nearly all occurred during nighttime flood tides (85%). Significant recruit–stock relationships occurred between observed settlement and fishery landings (r2 = 0.96) and effort (r2 = 0.94) in wet years and catch per unit effort (r2 = 0.98) during dry years. The timing of settlement pulses, but not the magnitude of annual settlement, was successfully predicted using the settlement model. These results suggest that blue crab fisheries in NC are influenced both by larval settlement and post‐settlement processes (freshwater inflow).

Effect of temperature on settlement and postsettlement survival in a barrens-forming sea urchin

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 413:69-80 (2010) - DOI: https://doi.org/10.3354/meps08684 Effect of temperature on settlement and postsettlement survival in a barrens-forming sea urchin José Carlos Hernández1,2,*, Sabrina Clemente1,2, Dominique Girard1, Ángel Pérez-Ruzafa3, Alberto Brito1 1Departamento de Biología Animal Biodiversidad, Ecología Marina y Conservación (Ciencias Marinas), Facultad de Biología, Universidad de La Laguna, La Laguna, Tenerife, islas Canarias 2Biology Department, Villanova University, 800 Lancaster Avenue, Villanova, Pennsylvania 19085, USA 3Departamento de Ecología e Hidrología, Universidad de Murcia, Murcia, España *Email: jocarher@ull.es ABSTRACT: Large populations of the sea urchin Diadema aff. antillarum occur in the east Atlantic archipelagos. The origin and stability of these large herbivorous populations are not well known. Variability in settlement, recruitment and adult urchin density were quantified and compared over a 7 yr period (2002 to 2008) within the Canary Islands. Variables that potentially affect urchin settlement such as gonad index, temperature and primary productivity (phytoplankton) were measured. Settlement was found to be high during warm years; temperatures >24°C induced important settlement pulses. Postsettlement survival in barren areas of Tenerife was exponentially and positively correlated with seawater temperature changes, while very low to 0 survival was found at macroalgal beds off El Hierro. Assuming that climate change predictions are correct, the strong positive relationship between thermal history and postsettlement survival of this ecologically important species suggests that barren grounds will persist as seawater warms up if no mass mortality occurs. This future scenario will have important implications for the stability of algal beds and coastal ecosystems as a whole. KEY WORDS: Diadema aff. antillarum · Larval settlement · Seawater warming · Recruitment · Habitat Full text in pdf format PreviousNextCite this article as: Hernández JC, Clemente S, Girard D, Pérez-Ruzafa Á, Brito A (2010) Effect of temperature on settlement and postsettlement survival in a barrens-forming sea urchin. Mar Ecol Prog Ser 413:69-80. https://doi.org/10.3354/meps08684 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 413. Online publication date: August 26, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Order of arrival affects competition in two reef fishes

Many communities experience repeated periods of colonization due to seasonally regenerating habitats or pulsed arrival of young-of-year. When an individual's persistence in a community depends upon the strength of competitive interactions, changes in the timing of arrival relative to the arrival of a competitor can modify competitive strength and, ultimately, establishment in the community. We investigated whether the strength of intracohort competitive interactions between recent settlers of the reef fishes Thalassoma hardwicke and T. quinquevittatum are dependent on the sequence and temporal separation of their arrival into communities. To achieve this, we manipulated the sequence and timing of arrival of each species onto experimental patch reefs by simulating settlement pulses and monitoring survival and aggressive interactions. Both species survived best in the absence of competitors, but when competitors were present, they did best when they arrived at the same time. Survival declined as each species entered the community progressively later than its competitor and as aggression by its competitor increased. Intraspecific effects of resident T. hardwicke were similar to interspecific effects. This study shows that the strength of competition depends not only on the identity of competitors, but also on the sequence and timing of their interactions, suggesting that when examining interaction strengths, it is important to identify temporal variability in the direction and magnitude of their effects. Furthermore, our findings provide empirical evidence for the importance of competitive lotteries in the maintenance of species diversity in demographically open marine systems.

Postlarval settlement of the spiny lobster Panulirus argus along the Caribbean coast of Mexico: Patterns, influence of physical factors, and possible sources of origin

We analyzed and compared 17 yr of monthly settlement indices of postlarvae of the Caribbean spiny lobster, Panulirus argus, at two locations on the Mexican Caribbean coast. Along this coast, the shelf is very narrow and the local circulation is driven by waves overtopping subtidal coral reef crests. Settlement occurred year‐round, but both locations showed annual rhythmicity, average seasonal maxima during the autumn, and similar interannual variation, reflecting the influence of mesoscale oceanographic processes on settlement. Significant correlations between settlement indices and a number of interrelated physical factors indicate that sea‐level variation is the main factor influencing settlement. There was further association between extreme settlement pulses and extreme weather events (tropical storms and hurricanes) during the late summer‐autumn. The autumn settlement peak indicates that large hurricane waves enhance the onshore transport of postlarvae that originated during the spring spawning peak. The geographic range of potential sources of larvae that settle along this coast during the periods of March‐April and September‐October, 5‐9 months after hatching (the estimated larval duration for P. argus), was investigated by stochastic simulations integrating passive trajectories backward in time using 6 yr of stable outputs from a well‐established numerical model for the entire Atlantic Ocean. Trajectories from the model indicate that the local population of P. argus depends to a large extent on larval supply from many potential source regions across and outside the Caribbean but that some larvae follow pathways that permit selfrecruitment.

HATCHING TIMES, LARVAL DURATION, SETTLEMENT AND LARVAL GROWTH OF PLAICE (PLEURONECTES PLATESSA (L.)) IN GALWAY BAY DETERMINED USING OTOLITH MICROSTRUCTURE

The otolith microstructure of juvenile plaice from two beaches in Galway Bay on the Irish west coast, Ballyloughaun and Silverstrand, was examined. Sampling was carried out monthly in the spring of 2003. The hatch date, larval duration, settlement date and larval growth rates were determined for 85 fish. Hatching occurred from late January to late March. The mean larval duration was 33 days, and fish settling earlier in the year had a longer larval life on Silverstrand. Settlement occurred between early March and late April. Settlement pulses closely corresponded to hatching times, indicating that hatching/settlement cohorts could exist for plaice in Galway Bay. Larval growth was consistent between locations assessed. There was a positive relationship between larval growth and hatch date, and an inverse relationship was noted between larval growth and larval duration. Results from this study were compared to studies from other European areas, and a relationship between larval duration and temperature was noted, with fish from cooler areas in Europe having longer larval durations. This is the first study of its kind on the west coast of Ireland and the first study within eastern Atlantic waters to focus on the early life history of plaice.

HATCHING TIMES, LARVAL DURATION, SETTLEMENT AND LARVAL GROWTH OF PLAICE (<i>PLEURONECTES PLATESSA</i> (L.)) IN GALWAY BAY DETERMINED USING OTOLITH MICROSTRUCTURE

The otolith microstructure of juvenile plaice from two beaches in Galway Bay on the Irish west coast, Ballyloughaun and Silverstrand, was examined. Sampling was carried out monthly in the spring of 2003. The hatch date, larval duration, settlement date and larval growth rates were determined for 85 fish. Hatching occurred from late January to late March. The mean larval duration was 33 days, and fish settling earlier in the year had a longer larval life on Silverstrand. Settlement occurred between early March and late April. Settlement pulses closely corresponded to hatching times, indicating that hatching/settlement cohorts could exist for plaice in Galway Bay. Larval growth was consistent between locations assessed. There was a positive relationship between larval growth and hatch date, and an inverse relationship was noted between larval growth and larval duration. Results from this study were compared to studies from other European areas, and a relationship between larval duration and temperature was noted, with fish from cooler areas in Europe having longer larval durations. This is the first study of its kind on the west coast of Ireland and the first study within eastern Atlantic waters to focus on the early life history of plaice.

Spatially heterogeneous, synchronous settlement of Chthamalus spp. larvae in northern Baja California

We evaluated the spatial variability in barnacle settlement at scales of 10s to 100s of meters (among-sites: 300 m; within-site: 30 m) along 1 km of coastline in the Bay of Todos Santos, northern Baja California, Mexico. Settlement of the intertidal barnacles Chthamalus spp. was moni- tored daily from April 1 to May 10, 2002, and thereafter every other day to September 18, 2002. Con- currently, temperature of the water column was measured every 15 min, and hourly wind speed and direction data were acquired from a nearby site. We identified 12 settlement pulses during our study, with all but 2 pulses showing significant differences in mean settlement at either or both spatial scales. Despite a high variability in numbers, settlement pulses were synchronous between sites. The occurrence of settlement pulses was significantly correlated with a rapid increase in the stratification of nearshore waters, but not correlated with sudden fluctuations in the direction of winds perpendic- ular to the shore. Sudden changes in the stratification of nearshore waters have been associated with the occurrence of internal tidal bores. Our results suggest that internal motions, more specifically internal tidal bores, could be an important mechanism for the onshore transport of larvae in the Bay of Todos Santos.

Population dynamics of the acorn barnacles, Tetraclita squamosa and Tetraclita japonica (Cirripedia: Balanomorpha), in Hong Kong

Tetraclita squamosa and Tetraclita japonica are common, intertidal barnacles. In Hong Kong, the population dynamics of the two species exhibited spatial and temporal variation on two semi-exposed shores. T. squamosa produced egg masses from May–June and annual settlement and recruitment occurred from June–July. In contrast, settlement and recruitment of T. japonica was sparse from March–May, but intensified in July–October. Mature gonads and egg masses were, however, only present from September–November, suggesting the larvae from the two settlement pulses originated from other locations as well as Hong Kong populations. Settlement intensity and post-recruitment mortality of the two species varied between sites, possibly due to spatial variation in free rock space, physical transport of larvae and abundance of food in the water column.

Temporal and spatial variability in settlement and recruitment of echinoderms in kelp beds and barrens in Nova Scotia

Settlement and recruitment of echinoderms were measured in 2 adjacent habitats: a kelp bed (Laminaria longicruris) and an echinoid (Strongylocentrotus droebachiensis)-dominated barren ground at each of 2 sites in the rocky subtidal zone (5 to 10 m depth) off Nova Scotia. Settle- ment was measured using artificial collectors made of 0.05 m 2 panels of plastic turf that were deployed 0.2 m (low) and 2.3 m (high) above the bottom in each habitat, and sampled bi-weekly from June to November 1992 to 1994. Distinct settlement pulses of ophiuroids (Ophiopholis aculeata and Ophiura spp.), asteroids (Asterias spp.) and an echinoid (S. droebachiensis) occurred between July and September of each year at both sites. Timing of settlement differed consistently among species in relation to differences in timing of spawning and larval development. The magnitude of each pulse varied between years and species, and the year of maximum settlement differed between species, suggesting that species-specific processes regulate settlement rather than general environmental conditions. Settlement of all species was greater at the more protected site, but between-habitat pat- terns were not consistent among species. Asteroids settled in greater numbers in and above kelp beds, whereas ophiuroids showed a trend toward greater settlement in the barrens. The echinoid S. droebachiensis also tended to settle more in barrens, although the difference was not statistically sig- nificant. This suggests that kelp beds do not exhibit a consistent or strong effect on settlement. Sam- pling settlement concurrently at different frequencies (3 to 48 d) gave different estimates of settle- ment, indicating a need for assessment of sampling artifacts (e.g. changes in collector quality, post-settlement mortality or migration) that can occur over longer deployment intervals. For most species sampled, settlement predicted recruit density in natural populations the following year. How- ever, the strength of the relationship varied between species, probably because of differing post- settlement processes.

Yearly and inter-island variation in the recruitment dynamics of the bluehead wrasse ( Thalassoma bifasciatum, Bloch)

The summer spawning and settlement patterns of Thalassoma bifasciatum (Bloch) in the Virgin Islands were back-calculated using daily increments of otoliths from recruits. Recruits were collected in July of each year of 1991–1993 from two sites on St. Croix, one on St. John, and one on Virgin Gorda. Spawning and settlement of T. bifasciatum exhibited slight lunar periodicity, with a tendency for more spawning around the last lunar quarter and higher settlement around the first lunar quarter. Settlement patterns failed to exhibit the brief episodes of intense settlement previously reported for this species, although weak pulses of settlement spread over several days were evident. Only in 1993 were settlement pulses coincident among sites, and this synchronous settlement was probably driven by a patch of water > 650 km 2 which was rich in larvae. In contrast to studies of this species at other sites, very few individuals recruiting to the Virgin Islands delayed metamorphosis. We conclude that settlement of this species in the Virgin Islands is patterned by the interaction of a relatively fixed larval duration with unpredictable oceanographic processes acting to transport larvae.

Local-scale Patterns of Larval Settlement in a Planktivorous Damselfish—Do They Predict Recruitment?

The degree to which local-scale spatial variation in larval supply predicted subsequent patterns in juvenile densities was examined for a planktivorous damselfish (Dascyllus trimaculatus) at Moorea, French Polynesia. D. trimaculatus young settle from the plankton to sea anemones, where they remain until becoming free-ranging adults. Daily settlement onto anemones without fish was estimated at several sites in a lagoon for one 14-day period and compared with patterns in the density of juveniles that had accrued after 10 successive settlement periods. Spatial variation attributable to larval supply occurred mostly among individual anemones, between the halves of each study site, and among different locations in the lagoon. In contrast, there was relatively little variation at any spatial scale in the density of young after 10 settlement pulses. Several mechanisms were examined that could account for the observed reduction in spatial variance through time, and three involving interactions with young already present were implicated. First, settlement was not independent of residents (the density of young already on an anemone); compared with the absence of fish, settlement was facilitated at low and inhibited at high densities of residents. Second, older juveniles moved among local anemones, and the immigration and emigration rates per capita varied with density on an anemone. Finally, per capita mortality during the first few days after settlement was density-dependent. These findings emphasize the critical need to distinguish among effects that arise from the delivery of larvae, the availability of appropriate habitat, and interactions that affect fish at settlement and immediately thereafter.

Patterns of lunar settlement and early recruitment in Caribbean reef fishes at Panam�

Lunar patterns of settlement of 15 Caribbean reef fishes were assessed from daily collections of newly arrived fishes from each of two small (A=63 and B=28 m2) patch reefs at Punta de San Blas (Panama) during two consecutive 2 yr periods (1984, 1985 and 1986, 1987). In 12 species settlement was lunar-cyclic, with a broad peak of activity around the new moon in 11 cases, and at first quarter in 1 species. However, 3 of these “lunar-cyclic” species displayed intermittent semilunar periodicity in settlement, with peaks around the quarter moons. One other species had a semilunar cycle of settlement with peaks at both quarter moons. Two species apparently lacked lunar settlement cycles. In some, but not all, paris of congeners: (a) monthly variation in settlement intensity was positively correlated, (b) the duration of settlement pulses in the same month consistently differed, and (c) the timing of settlement pulses in the same month often differed (although one species did not consistently arrive before the other). Recruitment of juvenile fish that survived to the end of the lunar cycle in which they settled was monitored monthly, concurrently with settlement, at a number of large sites scattered up to 2 km apart. The levels of variation in the amount of settlement and recruitment each month differed in only 2 of 17 cases, with settlement variation exceeding recruitment variation in only one of these. The intensity of recruitment was positively correlated with the intensity of settlement in 16 of 17 cases. In 15 of those 16 cases, R 2 values for linear regressions of recruitment on settlement exceeded R 2 values for curvilinear regressions that would indicate either increasing or decreasing recruitment success with increasing intensity of settlement. Thus it appears that variation in recruitment can be used to estimate variation in settlement, and that recruitment success may be density-independent.

The choice of a sieving mesh size in the quantitative assessment of marine macrobenthos: a necessary compromise between aims and constraints

The efficiency of several screen sizes in retaining macrofaunal organisms was tested in subtidal and estuarine intertidal communities. Although macrobenthos is currently defined as metazoans retained by a 1 mm mesh, sieving efficiency of this screen was found to vary between 20 and 70%, when compared with 0·5 mm meshes. Furthermore, 25–65% of specimens belonging to macrofaunal species passed through a 0·5 mm mesh, which shows that sieves finer than 1 or 0·5 mm clearly improve the quantitative collection of macrobenthos. Sieving efficiency displayed considerable seasonal variation due to settlement pulses, and differed according to phyla (generally, crustaceans > polychaetes > bivalves) and between species within a phylum. In some cases, species richness, the diversity index and evenness were influenced by sieve mesh size. Even though the subdivision of benthos in macro- and meiofauna reflects a real pattern, the boundary fixed between them at about 1·0–0·5 mm seems too clear-cut, due to the protracted stay of juvenile macrofauna in the meiofaunalsized class and a number of methodological factors and morphological features. The mesh selected for a particular task should be chosen with a clear idea of the study aims. It is thus suggested that a 1·0 mm mesh is sufficient only for biomass estimates or bionomics studies, whereas sieves with 0·1–0·2 mm mesh openings should be used in studies involving population dynamics of macrofauna, to provide an adequate estimate of abundances of individuals in small size classes.