Recruitment Bottleneck Research Articles

Acoustic estimates of abundance and distribution of spawning lake trout on Sheboygan Reef in Lake Michigan

Efforts to restore self-sustaining lake trout ( Salvelinus namaycush) populations in the Laurentian Great Lakes have had widespread success in Lake Superior; but in other Great Lakes, populations of lake trout are maintained by stocking. Recruitment bottlenecks may be present at a number of stages of the reproduction process. To study eggs and fry, it is necessary to identify spawning locations, which is difficult in deep water. Acoustic sampling can be used to rapidly locate aggregations of fish (like spawning lake trout), describe their distribution, and estimate their abundance. To assess these capabilities for application to lake trout, we conducted an acoustic survey covering 22 km 2 at Sheboygan Reef, a deep reef ( < 40 m summit) in southern Lake Michigan during fall 2005. Data collected with remotely operated vehicles (ROV) confirmed that fish were large lake trout, that lake trout were 1–2 m above bottom, and that spawning took place over specific habitat. Lake trout density exhibited a high degree of spatial structure (autocorrelation) up to a range of ∼ 190 m, and highest lake trout and egg densities occurred over rough substrates (rubble and cobble) at the shallowest depths sampled (36–42 m). Mean lake trout density in the area surveyed (∼ 2190 ha) was 5.8 fish/ha and the area surveyed contained an estimated 9500–16,000 large lake trout. Spatial aggregation in lake trout densities, similarity of depths and substrates at which high lake trout and egg densities occurred, and relatively low uncertainty in the lake trout density estimate indicate that acoustic sampling can be a useful complement to other sampling tools used in lake trout restoration research.

Effects of Salinity, Temperature, and Weight on the Survival of Young‐of‐Year Shortnose Sturgeon

AbstractAnthropogenic modifications of rivers and estuarine habitats have reduced the growth and recruitment of shortnose sturgeon Acipenser brevirostrum throughout its native range. This study used data from laboratory experiments to examine the individual and interactive effects of salinity, temperature, and fish weight on the survival of young‐of‐year shortnose sturgeon. The shortnose sturgeon in these experiments ranged from 56 to 187 mm in total length, from 0.4 to 42.8 g in weight, and from 66 to 144 d posthatch in age. Survival in 23°C water declined as salinity increased, but salinity tolerance increased with body size. The salinity lethal to 50% of the test fish after 48 h ranged from 14.8‰ to 20.9‰. Survival in freshwater declined as temperature increased, but temperature tolerance increased with body size. The temperature lethal to 50% of the test fish after 48 h ranged from 28.2°C to 30.7°C. In factorial experiments, salinity and temperature interacted significantly to affect young‐of‐year survival. The results of the factorial experiments were used to generate a predictive model in which the probability of survival was dependent on salinity, temperature, fish weight, and all interactions among the variables. The results of this study can be used to identify suitable habitats for young‐of‐year shortnose sturgeon. The results of this study also can be used to predict the recruitment bottlenecks that may result from additional anthropogenic disturbances to rivers in the southeastern United States.

Justice in translation: from bench to bedside in the developing world

As clinical research becomes increasingly global, however, individuals from low-income and middle-income countries (LMICs) have been recruited into small-scale, translational trials of novel technologies such as gene transfer. The distinctive ethical concerns that have arisen from such practices have received almost no attention. We consider four rationales for recruiting participants from LMICs into translational trials. First is fortuity. LMICs occasionally present investigators with a research opportunity that would otherwise be unavailable if they used volunteers from high-income countries. Thus, an Italian gene transfer study of severe combined immunodefi ciency disease associated with adenosine deaminase defi ciency (ADA-SCID) would not have been ethically permissible in high-income countries because participants could not use concurrent enzyme replacement therapy. Investigators moved participants who were unable to aff ord the high-income country standard of care from Palestine and Columbia to research centres in high-income countries. 2 Second is expediency; investigators sometimes recruit from LMICs to ease recruitment bottlenecks—eg, a trial of haemophilia gene transfer that sought participants in Brazil. 3

Employability enhancement of business graduates in China

PurposeThe purpose of this article is to provide an assessment of business educational reform requirements in China, in the light of current employability needs.Design/methodology/approachAfter introducing the problem the paper discusses labour market demands for business graduates. The paper continues with a thorough conceptualisation of the concept of employability and current notions on career development, and concludes with proposals for reforming the business education curriculum.FindingsEmployability enhancement should be an integrated activity that is undertaken in close collaboration between different parties, and to eliminate recruitment bottlenecks, policies that affect both graduates' suitability and labour requirements are needed.Practical implicationsThe paper is a useful source of information and advice for educators, employers, and business education students planning to invest in their future employability. The paper is meant to evoke discussion and to stimulate curriculum reform efforts.Originality/valueThis paper focuses on current labour market demands and offers insights for educational institutions engaged in optimizing curricula.

Rationale for Restocking the Eastern Baltic Cod Stock

The Danish Institute for Fisheries Research and Bornholm's Salmon Hatchery examined the potential for restocking Baltic cod (Gadus morhua callarias L.) in the eastern Baltic Sea. This cod population has adapted to the unique brackish water conditions where successful spawning depends on regular inflows of oxygenated saltwater from the North Sea. Hydrographical conditions are therefore considered to constitute the principal bottleneck for recruitment of this population. Successful recruitment is also dependent upon food availability and predation pressure from mainly herring (Clupea harengus L.) and sprat (Sprattus sprattus L.). A 2-to 3-month delay in the spawning period compared to 20–30 years ago has altered feeding conditions and predation susceptibility in a way that may have exacerbated the decline in recruitment. Producing and releasing cod larvae during spring would mimic the spawning period recorded in previous times and would coincide with the spring peak in copepod production. An evaluation of 3 different release scenarios showed that a release of 474 million first-feeding larvae over 5 months would enhance the average population of 2-year-olds by 10% and be biologically and economically the most feasible scenario.

A multi-scale approach in the study of plant regeneration: Finding bottlenecks is not enough

Using a multi-scale design, we investigated the spatio-temporal variability and concordance across recruitment stages of the Mediterranean fleshy-fruited shrub Daphne gnidium. The patterns of seed rain, post-dispersal seed predation, seedling emergence, survival and growth were studied in two populations in contrasting conditions, in an olive grove and in a dune system, in Mallorca (Balearic Islands) during two consecutive seasons. We considered 20 sites (fine scale) in four microhabitats (medium scale) in each population (large scale). Most seeds were dropped under parent plants, especially in the population with presumably less seed dispersers. Post-dispersal seed predation (ca. 90%) represented the most important bottleneck in plant recruitment in all microhabitats at both populations. Seedling emergence was also similar at both populations although seedling survival was higher in the olive grove. One year after of emergence, seedlings under shrubs were taller – though not necessarily had more leaves – than in the other microhabitats. There was a positive and consistent effect of seed size across microhabitats on seedling emergence, survival and growth (maximum length) although its intensity varied between populations and years. Concordance between seed rain and seedling recruitment was affected by the heterogeneity of population-dependent forces acting upon each recruitment stage. There were stages, which represented a strong bottleneck but were little discordant with seed rain. By contrast, other weak bottlenecks were hardly discordant with seed rain. Our study stresses the importance of considering simultaneously the critical stages (i.e., bottlenecks) and the concordance between seed dispersal and post-dispersal stages in plant recruitment studies.

Cold Summer Temperature Regimes Cause a Recruitment Bottleneck in Age‐0 Colorado River Cutthroat Trout Reared in Laboratory Streams

AbstractNative salmonids are increasingly restricted to upstream habitats that may be too cold to sustain recruitment, and recruitment limitation owing to cold temperatures is a main hypothesis to explain translocation failures of native cutthroat trout Oncorhynchus clarkii in high‐elevation streams in the southern Rocky Mountains. We subjected Colorado River cutthroat trout O. c. pleuriticus fry to one of three temperature regimes (cold, intermediate, or warm), which averaged 7.0, 8.5, and 10.0°C during the warmest summer month, in each of 2 years. The regimes mimicked those of natural streams where translocated fish had died out or produced populations of cutthroat trout of low or high abundance. The mean fry survival rate from hatching through swim‐up was high during both experiments, ranging from 97% in the warm regime to 85% in the cold. After swim‐up, fry in the warm regime grew more than 60% more on average than those in the cold regime by the onset of winter in 2003 during a 22‐week period. Survival rates through midwinter were also higher in the warm regime (76%) than in the intermediate (62%) and cold (29%) regimes. A similar pattern of temperature‐related growth and mortality was apparent in the 2004 experiment (12 weeks), in which survival rates to early winter ranged from 44% in the warm regime to 10% in the cold. Most mortality in all three treatments occurred during a recruitment bottleneck encompassing a 4‐6‐week period following swim‐up. Analyses of size and percent dry weight indicated that the energy content of fry after swim‐up was lower in colder regimes and declined during the recruitment bottleneck in all three regimes. Our findings indicate a strong recruitment bottleneck after swim‐up when temperature‐related energy deficits apparently cause significant mortality. Managers may increase the viability of these translocated populations by selecting sites that accumulate more than 900 degree‐days during summer.

Plant recruitment bottlenecks in temperate forest fragments: seed limitation and insect herbivory

Numerous studies have documented declines in plant diversity in response to habitat loss in fragmented landscapes. However, determining the mechanisms that lead to species loss is challenging using solely a correlative approach. Here we link correlative assessments of plant community composition with seed additions for a focal species to test the hypothesis that distributions of forests plants within a fragmented landscape are limited by seed dispersal. Woody plant species richness of fragments declined as fragments (n=26) became more isolated by agricultural fields. We predicted that if these isolation effects were driven by poor dispersal rather than other effects associated with habitat loss, then plants should vary in their response to isolation in relation to their seed size (i.e., stronger effects for plants with larger seeds). As predicted under this dispersal limitation hypothesis, sensitivity of bird-dispersed shrubs to isolation was related to their seed mass, with species with heavy seeds (e.g., Lindera benzoin) exhibiting stronger declines in presence across isolation gradients than species with light seeds. Seed addition experiments were performed for Lindera benzoin in two high isolation forest fragments (nearest neighbor mean distance=803 m) where Lindera was naturally absent, and two low isolation fragments (nearest neighbor mean distance=218 m) with naturally occurring Lindera populations. Seed addition and control plots (n=50 1 m2 plots per fragment) were monitored for 13 censuses over 3 years. Across all four fragments, seed additions resulted in significant increases in Lindera seedling recruitment with no differences in final seedling establishment among fragments. However, insect herbivory was higher on Lindera seedlings in high isolation compared to low isolation fragments and was negatively correlated with seedling survival over some years. Consistent with prior work, our results confirm that seed dispersal plays a significant role in affecting plant diversity in fragmented landscapes. However, results also suggest the need for a better understanding of how additional processes, such as herbivory, may be altered as habitat is lost and what effects such changes have for forest plants.

Implications of Thiamine Deficiency in Great Lakes Salmonines

AbstractOur recent experimental work and ecoepizootiological assessments provide mechanistic data supporting a plausible hypothesis for an association between a prey base comprised of a large biomass of nonnative alewives Alosa pseudoharengus and the recruitment difficulties currently experienced by Great Lakes salmonines. We hypothesize that the thiamine deficiency induced by alewives, a species harboring high thiaminase activity, represents an ongoing cause of fry and adult mortality in salmonines. Overall ramifications of the thiamine deficiency on recruitment have not been firmly established but may represent a substantial bottleneck for natural recruitment in feral salmonine populations in the Great Lakes.

Ontogenetic dietary shifts of fishes in an Australian floodplain river

The early life of fishes is marked by rapid development when individuals are thought to optimise their success through ontogeny by altering ecological niches and resources. Since most fish larvae are thought to require small prey items at first feeding, competition for potentially limiting food resources may occur between species creating a juvenile bottleneck, which may, in turn, influence future recruitment strength. The diets of the early life stages of most Australian freshwater fish are poorly known. The present study investigated the ontogenetic dietary patterns of six species of fish in an Australian floodplain river. A large proportion of first-feeding larvae of three species (Murray cod, Australian smelt and carp) were able to feed externally while still retaining their yolk sac. All species demonstrated major dietary shifts from newly hatched larvae through development into juvenile stages and adulthood. Only a few minor overlaps in diet were found, with greater overlaps commonly occurring between sequential stages of the same species, reflecting subtle ontogenetic changes. Despite two co-occurrences of an introduced and a native species using the same rearing habitat as larvae, dietary preferences did not significantly overlap, suggesting that a recruitment bottleneck caused by competition for food resources is unlikely for these species.

Lake Trout Reproduction in Lake Champlain

Native lake trout Salvelinus namaycush were driven to extirpation in Lake Champlain in the early 1900s. Possible causes include overharvest, predation on adults by sea lamprey Petromyzon marinus, and predation on fry by rainbow smelt Osmerus mordax. Efforts to restore a lake trout fishery began in 1972 when a coordinated stocking program was initiated. Attempts to control sea lamprey populations began in 1990. Despite these management actions, reproduction by stocked fish has not resulted in large, naturally produced year-classes. This is the first formal study to quantitatively assess the level of natural reproduction by lake trout in Lake Champlain. In 2000–2002, we located 14 potential lake trout spawning sites and evaluated the habitat characteristics and level of spawning activity at each site. Passive egg collectors revealed that eggs were deposited at 8 of 14 sites, egg abundance ranging from 1.9 to 9,623 eggs/m2. In 2001 and 2002, lake trout fry were collected in emergent fry traps at three of five sites; catch per unit effort ranged from 0.08 to 2.38 fry·trap−1·d−1. We were unable to collect naturally produced juvenile lake trout through bottom trawling. We also examined adult lake trout size structure and abundance data collected annually by state agencies to determine whether there was a trend in the percentage of unmarked lake trout in the population. The percentage of unclipped lake trout in Lake Champlain decreased steadily from 1982 (7.4%) to 1988 (1.7%), was variable from 1989 to 1991, increased from 1992 (2.6%) to a maximum in 2000 (10.4%), and then decreased to 5.7% in 2001. The high levels of egg and fry abundance, the failure to collect lake-produced juveniles, and the low percentage of unclipped adult fish all suggest that a recruitment bottleneck is present during the postemergent fry life stage.

Lake Ontario: food web dynamics in a changing ecosystem (1970–2000)

We examined stressors that have led to profound ecological changes in the Lake Ontario ecosystem and its fish community since 1970. The most notable changes have been reductions in phosphorus loading, invasion by Dreissena spp., fisheries management through stocking of exotic salmonids and control of sea lamprey (Petromyzon marinus), and fish harvest by anglers and double-crested cormorants (Phalacrocorax auritus). The response to these stressors has led to (i) declines in both algal photosynthesis and epilimnetic zooplankton production, (ii) decreases in alewife (Alosa pseudoharengus) abundance, (iii) declines in native Diporeia and lake whitefish (Coregonus clupeaformis), (iv) behavioral shifts in alewife spatial distribution benefitting native lake trout (Salvelinus namaycush), threespine stickleback (Gasterosteus aculeatus), and emerald shiner (Notropis atherinoides) populations, (v) dramatic increases in water clarity, (vi) predation impacts by cormorants on select fish species, and (vii) lake trout recruitment bottlenecks associated with alewife-induced thiamine deficiency. We expect stressor responses associated with anthropogenic forces like exotic species invasions and global climate warming to continue to impact the Lake Ontario ecosystem in the future and recommend continuous long-term ecological studies to enhance scientific understanding and management of this important resource.

The mental health practitioner – bypassing the recruitment bottleneck

Lack of resources has been a major restriction on the development of mental health services. However, even with the resources currently available there are insufficient numbers of trained medical, nursing, occupational therapy, psychology and social work staff to maintain services to adequate levels in many areas. This seriously interferes with provision of services, especially in acute wards but also in other areas. It certainly restricts developments and the use of skills attained through training (e.g. from THORN psychosocial intervention courses (Gournay &amp; Birley, 1998)). The introduction of crisis resolution and early intervention teams, as described in the NHS Implementation Guide (Department of Health, 2001a), looks likely to simply deprive in-patient wards and community teams of staff, making the new teams ineffective through lack of core services. This will occur directly by recruitment of staff from them, or competitively through taking new entrants from nursing and social work programmes. Solutions proposed have included increasing numbers of support workers and administration staff; recruitment from abroad; or increased delegation of tasks, but there remains a need for more appropriately-trained professional staff.

Through the Gauntlet Again: Demographic Restructuring of American Shad by Migration

Long-distance migration in fishes is one of the more spectacular examples of ontogenetic niche switching, yet details are often obscured because of the difficulty in making direct observations on individual migrants. Juvenile emigration and ultimate recruitment to the spawning population were studied retrospectively in American shad (Alosa sapidissima), to look for patterns of size-biased survivorship as a consequence of migration. Adults were collected on or near the spawning grounds in the Hudson River, New York, USA, in spring 1995. Their early life histories were reconstructed by measuring strontium and calcium concentrations in the inner portions of their otoliths, which were deposited when the fish were young-of-the-year (YOY). Sudden elevations of the Sr:Ca ratio were interpreted as seaward migration events, and the physical location of these elevations on the otolith could be related to the age and size at which the fish first emigrated from fresh water. Recruited adults showed distributions of first emigration that were bimodal with respect to size and age, and these patterns were repeated in three of four year classes analyzed. In 1989 and 1990, years when YOY data were available, size distributions of apparently emigrating YOY do not agree with the distributions displayed in the otoliths of returning adults from the same year classes, suggesting that differential mortality had occurred, which was dependent on the timing of emigration. Detailed analysis was possible for the 1990 year class, which showed strong evidence in both sexes of at least two major recruitment bottlenecks—one affecting larvae hatched early, and the second affecting emigrating juveniles hatched from late cohorts. Potential causes of differential mortality on emigrating juveniles include the presence of large predators outside of the Hudson, as well as the presence of hypoxic waters into which the fish may have entered. This study indicates that long-distance migration of juveniles can be a strong factor that restructures the demography of a year class of fish.

Spatial pattern of Quercus ilex and Quercus pubescens recruitment in Pinus halepensis dominated woodlands

Abstract. European Mediterranean landscapes have undergone changes in structure in recent years as a result of widespread agricultural land abandonment and cessation of silvicultural regimes. Studies concerning the regeneration dynamics of dominant forest species have become critical to the prediction of future landscape trends in these changing forest stands. Quercus ilex (holm oak) and Q. pubescens (downy oak) are considered to be the terminal point of secondary succession in extensive areas of the Mediterranean region. Recent studies, however, have suggested the existence of recruitment bottlenecks in oak genet populations as a result of current management regimes. In this study, we present evidence of the successful establishment of Q. ilex and Q. pubescens in Pinus halepensis (Aleppo pine) woodlands. We investigate the distribution patterns and spatial relationships among oak recruits and resident pines. Established P. halepensis is randomly distributed throughout the study area. Oak seedlings are positively associated with pine trees, suggesting that P. halepensis individuals provide safe sites for oak genet recruitment. We show that spatial patterns of recruitment are in agreement with the general model of spatial segregation described for other Mediterranean plant communities, with seeder species colonizing large openings after disturbance, followed by a more aggregated recruitment of resprouter species.

Factors controlling the establishment ofHelianthemum squamatum,an endemic gypsophile of semi-arid Spain

1 The restriction of some vascular plants to soils high in gypsum under arid or semi-arid climates has been reported by many authors in different parts of the world. Factors controlling the presence of gypsophytes on these soils are far from being definitively explained. 2 The establishment of Helianthemum squamatum, an Iberian gypsophyte, was investigated during a 2-year period in a typical semi-arid gypsum gradient landscape in central Spain that included the three community belts where this plant can grow. 3 More than 500 seedlings were tagged. Their growth and survival were monitored periodically (23 census dates). Over 80% of the recorded seedlings emerged during the first year, which followed a severe and long drought period. Germination occurred principally in winter (over 85%), although seeds germinated sporadically even in late May. Emergence was clearly spatially confined, with seedlings appearing only in the summit area, which was characterized by gypsum outcrops (160 per m2), and at the boundary between gypsum slope and alluvial piedmont (273 per m2). Seedlings were very scarce in lower areas (Lygeum spartum and Artemisia herba-alba communities) and absent from the steeper gypsum slope. 4 Survival was size-dependent: larger plants had a better chance of surviving the first summer, which is the bottle-neck for recruitment. We detected few differences in the final survival percentage after the first year for cohorts or zones, and survival curves were also similar, except at the boundary between gypsum slope and piedmont. However, seedlings emerging in 1997 had a higher survival percentage than those emerging in 1996. 5 Proximity to a seed source was the most relevant predictor of emergence. This may be related to the presence of mucilaginous coats in H. squamatum seeds, which may enable plants to become established on crusted soils. 6 Our results seem to link gypsophily with some properties of the surface crust, which is thought to contribute to the restrictive behaviour of such soils. We detected seedlings emerging on soils with very high surface strength, but also in alluvial soils. This suggests that H. squamatum can penetrate hard soils but also has the potential to grow on less restrictive soils. Competition with annuals may exclude H. squamatum on bottom flats.

Size-structured diel use of river banks by fish

We examined the role of body size in the diel use of river banks by fish and the abundance of piscivorous fishes to determine if predation pressure influences diel habitat use. We hypothesised that the mean standard length (SL) of non-piscivorous fish would decrease at night. Fish were sampled via point sampling by electrofishing every 3-4 h over three 24-h periods (late August 1992, early September 1993, late May 1994) along two adjacent stretches of bank (shallow sand, steep boulder) of the River Morava, Czech Republic. Gill netting in mid-channel adjacent to the sand bank was also used in 1993 to determine predator abundance. Analysis of variance and size-class ordination revealed higher numbers of smaller fish were captured at night along the banks than during the day, being in most cases significant. In almost all cases (all years and along all bank types), the standard length (SL) of non-piscivorous fishes was significantly lower during hours of darkness, in particular near midnight. Conversely, the SL of the non-piscivorous fishes in August 1993 was significantly higher in the gill nets during hours of darkness. Size class ordination and electivity indices revealed that bitterling Rhodeus sericeus in August 1992 were distributed according to size along the sand bank (smaller size classes) and the boulder bank (larger size classes); also, equivalent or adjacent size classes of Rutilus rutilus (41-50, 51-60, 61-70, 71-80, 81-105 mm SL), a large bodied species, and Gobio albipinnatus (26-50, 51-60, 61-70, 71-80, 81-90mm SL), a small-bodied species, co-exploited both types of river bank, moving to the sand bank at night, a possible example of recruitment bottleneck. In May 1994, smaller size classes of Leuciscus leuciscus (78-84, 85-90, 90-108 mm SL) and L. cephalus (63-99, 100-149, ≥ 150 mm SL) preferred the sand and boulder banks, respectively, with the largest sizes of the two species occurring together more often than expected along both banks. The results generally support our hypothesis, suggesting that shallow river shorelines represent important night-time refuges from predation for the smaller size classes of fish, thus constituting crucial life-history habitat important for the recruitment of many riverine fishes.

INTRASPECIFIC VARIATIONS IN δ13C INDICATE ONTOGENETIC DIET CHANGES IN DEPOSIT-FEEDING POLYCHAETES

Many species change diets during development. Often, these ontogenetic changes are discrete and coincide with metamorphosis (e.g., amphibians), but more gradual niche changes can occur during growth. Identifying nondiscrete changes in diet and understanding their implications at the population and community levels are especially difficult for ecologists who study detritivores and other species that have poorly characterized diets. Theory and several lines of evidence suggest, however, that benthic juveniles of species that deposit feed as adults may be unable to meet their nutritional demands by deposit feeding. To reject the null hypothesis that both juveniles and adults of deposit-feeding species assimilate the same diet and to infer ontogenetic changes in diet, I used stable carbon isotopes as a natural diet tracer. I quantified body-size-dependent variations in the δ13C of four species of tentaculate, surface-deposit-feeding polychaetes: the ampharetid Hobsonia florida and the spionids Pseudopolydora kempi japonica, Polydora cornuta, and Pygospio elegans. In addition to worm tissues, I measured the isotopic compositions of the most likely primary producers at each field site (benthic diatoms, macroalgae, and saltmarsh grasses) to predict the worms’ carbon sources. All species showed significant size-dependent variations in δ13C. Furthermore, populations of P. kempi japonica at two different sandflats had similar ontogenetic trends despite isotopic differences in available foods at each site. Individuals fed a fixed diet in the laboratory, however, did not show significant size-dependent variation in δ13C, leaving ontogenetic changes in diet as the most parsimonious explanation of the field data. Regression analyses indicated that the gradual change in δ13C with body size was nonlinear, with most of the change in δ13C occurring before individuals reach sexual maturity. The complex life cycle of these species, therefore, includes both a radical change in niche when larvae metamorphose to juveniles and a gradual niche shift as benthic juveniles grow. The isotopic data indicate that the smallest juveniles assimilated much of their carbon from benthic diatoms (δ13C ≈ −20‰), while adults assimilated most of their carbon from detritus derived from macroalgae (δ13C ≈ −9‰) or saltmarsh grasses (δ13C ≈ −14‰). Because abundances of benthic diatoms or other high-quality components of sediment are more variable and more likely to be in limiting supply than detritus or bulk sedimentary organics, populations of species that deposit feed as adults may experience food-related recruitment bottlenecks during the juvenile stage.

Spectrofluorometric quantification of neutral and polar lipids suggests a food‐related recruitment bottleneck for juveniles of a deposit‐feeding polychaete population

A growing body of evidence suggests that juveniles of species that deposit feed as adults are more susceptible to food limitation than are conspecific adults. To quantify ontogenetic variations in the nutritional condition of the depositfeeding polychaete Pseudopolydora kempi japonica, I modified existing spectrofluorometric assays of neutral and polar lipids. The hydrophobic fluorochrome Nile red fluoresces yellow‐orange when bound to neutral lipids and red‐orange when bound to polar lipids. The ratio of neutral lipids (energy storage) to polar lipids (cell membranes) provides an index of nutritional condition that is scaled to body size. For individual P. kempi japonica ranging in length from 2 to 12 mm, I quantified the florescence from each lipid class and analyzed sizedependent variations in the neutral/polar lipid ratio. P. kempi japonica showed a size‐dependent increase in the neutral/polar lipid ratio up to a body length of −6–8 mm; little size‐dependent change in the ratio occurred beyond this size. Ontogenetic changes in both particle‐size selection and the assimilated diet of P. kempi japonica display size‐dependent patterns that are similar to the lipid data. Together, the three lines of evidence suggest that P. kempi japonica juveniles experience a food‐related recruitment bottleneck until they grow to −6–8 mm. Once juveniles grow beyond this size, they appear to forage like adults and achieve adult levels of neutral lipid. Because deposit‐feeding benthos ingest a low‐quality and nutritionally dilute diet that will be difficult for small juveniles to digest, ontogenetic niche shifts and food‐related recruitment bottlenecks during the juvenile stage may be common in the population ecology of species that deposit feed as adults.

Aspects of the Dynamics of the High Forest at Langebos (Alexandria Forest Type)

SYNOPSIS All of the four dominant canopy species in the tall forest at Langebos appear to have recruitment bottlenecks (i.e. a relative absence of small stems). All apparently need large gaps to recruit, except Podocarpus falcatus which is shade-tolerant. Compared with the Knysna Forest there are very few stems per hectare at Langebos, especially in smaller size classes, although total basal area is similar. This relative lack of small stems is apparently due to a relative lack of shade-tolerant canopy species.