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1.
Intertidal population dynamics are driven by a complex series of processes, including larval supply and the possibility of larval predation by benthic animals such as filter-feeders. We hypothesised that cannibalism by adults could play a major role in the population connectivity of mussel populations by removing larvae as they attempt to settle in the adult habitat. Specifically, we tested hypotheses that consumption of mussel larvae by adults removes a significant proportion of potential settlers and is influenced by both settlement intensity and tidal state (flooding or ebbing). Predation of mussel larvae by adult mussels was investigated on incoming and ebbing tides during four spring tides by analysing the gut contents of adult Perna perna and Mytilus galloprovincialis collected from the low intertidal mussel zone between October 2005 and January 2006. Consumption rates were then compared with estimates of successful settler densities on natural beds. The results showed that mortality of competent mussel larvae through adult ingestion removes up to 77% a of potential settlers. Rates of larval consumption were highest during months of intense settlement, suggesting that mussels feed opportunistically, filtering a relatively fixed volume of water and removing particles, including larvae, in proportion to their densities in the water. Rates of larviphagy were also higher during receding than incoming tides. We suggest that this is due to changes in larval density or, more probably, in adult filtration efficiency that are related to the state of the tide. Despite significant effects of both tidal state and settlement intensity on rates of larval ingestion, neither had a significant effect on the proportion of potential settlers removed. During settlement more than half of all potential settlers are lost through cannibalism, with potentially serious consequences for population maintenance. The results highlight the paradoxical nature of the evolution of settlement mechanisms in mussels, which must balance the advantages of settlement in habitats favourable to adults against the consumption of larvae by adults.  相似文献   

2.
Bivalve populations from inshore waters often accommodate a diverse trematode fauna that may have a variety of effects on host specimens. In particular, larval trematodes that grow or reproduce within their host are known to be severe pathogens, whereas trematodes utilising bivalves only for encystment are thought to be relatively benign. Yet this may depend on the environmental conditions, and it can be expected that such trematodes in concert with other stress agents can be detrimental to host organisms. To examine the impact of such larval trematodes on hosts subjected to stress, we studied the digenetic trematode Himasthla elongata and one of its second intermediate hosts, the bivalve Cerastoderma edule. Experimentally infected cockles and non-infected cockles were exposed to oxygen depletion, whereupon we measured their burrowing ability and survivorship. After 30 h of hypoxia, the survival of infected cockles was significantly reduced compared to non-infected cockles, whereas no effect of parasites on cockles under normoxic conditions was found. In addition, parasites tended to reduce the burrowing ability of cockles exposed to hypoxia but the effect was not clear. The effect of parasites and possible ecological consequences are discussed and it is suggested that the combined effects of parasites and oxygen deficiency may explain some hitherto unexplained cases of mass mortalities in bivalve populations.  相似文献   

3.
对流行病学分析显示大珠母贝幼虫群体的死亡是以暴发性方式发生,从胚胎发育前期至变态期,幼虫存在两个死亡高峰,分别位于壳顶期之前或壳顶期和壳顶后期.大珠母贝幼虫群体的死亡率与RLO感染之间存在显著的相关性,在RLO感染的高峰之后均相应伴随幼虫群体死亡率的高峰,随着RLO感染率和感染强度的降低,幼虫群体死亡率也明显降低.在TEM下未发现未受精卵母细胞、受精卵和胚胎早期幼虫感染RLO,最先出现RLO包涵体阳性感染的时期为24h尚未摄食的D形期幼虫,故认为RLO可能不是通过垂直传播,而是通过外界接触感染的方式感染宿主,贝体感染至发病死亡存在潜伏期.  相似文献   

4.
We propose a new hypothesis, the Oscillating Control Hypothesis (OCH), which predicts that pelagic ecosystem function in the southeastern Bering Sea will alternate between primarily bottom-up control in cold regimes and primarily top-down control in warm regimes. The timing of spring primary production is determined predominately by the timing of ice retreat. Late ice retreat (late March or later) leads to an early, ice-associated bloom in cold water (e.g., 1995, 1997, 1999), whereas no ice, or early ice retreat before mid-March, leads to an open-water bloom in May or June in warm water (e.g., 1996, 1998, 2000). Zooplankton populations are not closely coupled to the spring bloom, but are sensitive to water temperature. In years when the spring bloom occurs in cold water, low temperatures limit the production of zooplankton, the survival of larval/juvenile fish, and their recruitment into the populations of species of large piscivorous fish, such as walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus) and arrowtooth flounder (Atheresthes stomias). When continued over decadal scales, this will lead to bottom-up limitation and a decreased biomass of piscivorous fish. Alternatively, in periods when the bloom occurs in warm water, zooplankton populations should grow rapidly, providing plentiful prey for larval and juvenile fish. Abundant zooplankton will support strong recruitment of fish and will lead to abundant predatory fish that control forage fish, including, in the case of pollock, their own juveniles. Piscivorous marine birds and pinnipeds may achieve higher production of young and survival in cold regimes, when there is less competition from large piscivorous fish for cold-water forage fish such as capelin (Mallotus villosus). Piscivorous seabirds and pinnipeds also may be expected to have high productivity in periods of transition from cold regimes to warm regimes, when young of large predatory species of fish are numerous enough to provide forage. The OCH predicts that the ability of large predatory fish populations to sustain fishing pressure will vary between warm and cold regimes.The OCH points to the importance of the timing of ice retreat and water temperatures during the spring bloom for the productivity of zooplankton, and the degree and direction of coupling between zooplankton and forage fish. Forage fish (e.g., juvenile pollock, capelin, Pacific herring [Clupea pallasii]) are key prey for adult pollock and other apex predators. In the southeastern Bering Sea, important changes in the biota since the mid-1970s include a marked increase in the biomass of large piscivorous fish and a concurrent decline in the biomass of forage fish, including age-1 walleye pollock, particularly over the southern portion of the shelf. Populations of northern fur seals (Callorhinus ursinus) and seabirds such as kittiwakes (Rissa spp.) at the Pribilof Islands have declined, most probably in response to a diminished prey base. The available evidence suggests that these changes are unlikely the result of a decrease in total annual new primary production, though the possibility of reduced post-bloom production during summer remains. An ecosystem approach to management of the Bering Sea and its fisheries is of great importance if all of the ecosystem components valued by society are to thrive. Cognizance of how climate regimes may alter relationships within this ecosystem will facilitate reaching that goal.  相似文献   

5.
Pagurus maclaughlinae is the most common hermit in the Indian River Lagoon System. Wide variations in lagoonal salinity make it likely that P. maclaughlinae is euryhaline and that other hermit species in the area are more stenohaline, at least in some stages of their life histories. In a study of salinity tolerance, crabs were held unfed at salinities of 5–50 (25 control) for up to 30 days. Based on survivorship curves, P. maclaughlinae tolerated acute exposure to salinities of 10–45 for up to 18 days, and survivorship up to 30 days at 20–45 equaled or exceeded survivorship of the control. In a study of acclimation, the osmotic pressure of hemolymph was measured after crabs were held in the laboratory for 12, 48, and 96 h acutely exposed to salinities of 10–45. Paired t-tests revealed that the crabs weakly hyperregulated their hemolymph at 45–154 mOsmol above the external medium at all salinities and sampling times, and the osmotic differential of their hemolymph was fully acclimated by 96 h. In a third study, acclimatization of hemolymph was studied on crabs at four field sites that differed in their recent salinity histories. Field-collected crabs weakly regulated their hemolymph 72–84 mOsmol above the external medium at all sites sampled. Performance did not differ by site. The range of salinity tolerance and acclimation of hemolymph of P. maclaughlinae partly explain their wide distribution, and the consistent osmotic differential of its hemolymph indicates that the osmoregulatory ability of this small-bodied species is conserved in populations throughout the lagoon. Although some other larger-bodied hermit species in the region are euryhaline as adults, their tendency to hyperregulate strongly at low salinities possibly adds an energetic burden that, along with their less euryhaline long-lived larvae, might exclude them from the lagoon. Salinity tolerance of larval P. maclaughlinae has yet to be studied.  相似文献   

6.
The size structure of coral populations is the culmination of key demographic events, including recruitment, mortality and growth, thereby providing important insights to recent ecological dynamics. Importantly, the size structure of corals reflects both intrinsic (inherent life-history characteristics) and extrinsic (enhanced mortality due to chronic or acute disturbances) forcing on local populations, enabling post-hoc assessment of spatial and taxonomic differences in susceptibility to disturbance. This study examined the size structure of four locally abundant corals (Acropora downingi, Favia pallida, Platygyra daedalea, and massive Porites spp.) in two regions of the Persian Gulf: the southern Gulf (Dubai and Abu Dhabi) and eastern Gulf (western Musandam). Significant and consistent differences were apparent in mean colony sizes and size-distributions between regions. All corals in the southern Gulf were significantly smaller, and their size structure positively skewed and relatively more leptokurtic (i.e., peaky) compared to corals in the eastern Gulf. Sea surface temperatures, salinity, and the recent frequency of mass bleaching are all higher, in the southern Gulf, suggesting higher mortality rates and/or slower growth in these populations. Differences in size structure between locations were more pronounced than differences between species at each location, suggesting that extreme differences in environmental conditions and disturbance events have a greater influence on population dynamics in the Gulf than inherent differences in their life-history characteristics.  相似文献   

7.
The Foveaux Strait oyster (Ostrea chilensis) fishery in southern New Zealand comprises many localised populations (oyster beds) that have survived disease mortality and 150 years of fishing. The reproductive biology of O. chilensis underlies the assumption that these populations are self-recruiting. A three-year study using passive, artificial collectors deployed in a gradient design around an isolated natal population investigated the hypothesis of self-recruitment. Spat settlement patterns measured the distributions of competent larvae as indicators of dispersal. This research also investigated the relationship between settler and brooder densities. Settler densities were not predicted by direction along or across the current, distance from the focal population, or by brooder densities. Settlement was widespread, and settlement patterns imply greater dispersal and larval mixing than previously reported. The swift currents and variable pelagic larval duration may enhance mixing and connectivity between populations. Demographically open recruitment should provide some resilience to disease mortality and fishing.  相似文献   

8.
滨海电厂温排水停止排放以及季节更替、寒潮来袭等现象会引起海水温度降低,从而对鱼类等变温生物产生冷冲击效应。本研究通过模拟水温骤降情形,以孵化率、死亡率、畸形率等为指标,探讨了温度骤降对大黄鱼早期发育阶段(鱼卵和仔鱼)的冷冲击作用。研究发现,大黄鱼仔鱼(3日龄)对温度骤降的敏感性略高于鱼卵。在大黄鱼鱼卵和仔鱼的适温范围内,当水温由22 ℃骤降至19 ℃或16 ℃时,鱼卵的孵化率和死亡率无明显变化,而胚胎发育和仔鱼的生长发育均减缓,仔鱼的死亡率提高;水温超出适温范围,由22 ℃骤降至13 ℃或10 ℃时,对鱼卵和仔鱼造成致命的冷冲击伤害,48 h累积死亡率分别为84.6%~100%和72.1%~98.2%。由此推测:当水温骤降超出适温范围后,大黄鱼的早期发育阶段遭受致命的冷冲击伤害,从而影响种群补充和年龄结构。  相似文献   

9.
The estuarine brachyuran crab Neohelice granulata export their larvae from the parental intertidal population of the Mar Chiquita coastal lagoon, and probably other populations, to marine waters. The degree of larval dispersion or self-recruitment of populations is unknown. We evaluated the presence of all larval stages of N. granulata in coastal waters of Argentina between 37.9° and 35.8° S, at two different spatial scales: a broad scale of tens to hundreds of kilometers from the Río de la Plata estuary in the north, to Mar Chiquita lagoon in the south, and a small scale of hundreds of meters to some kilometers around the mouth of Mar Chiquita, during spring and summer. Additionally, we registered the larval composition and density at San Clemente creek population, in Samborombon Bay (Río de la Plata estuary), every 3 h along a 30-hour period. Evidence indicates that larval release of N. granulata is temporally synchronized with nocturnal ebb tides and all development from Zoea I to Zoea IV occur in areas close to the parental population, even with very different oceanographic characteristics. A possible mechanism based on salinity selection and wind-driven transport is proposed for such behavior, and some considerations related to the connectivity of present populations are made.  相似文献   

10.
The effect of temporal variation in growth rate on per-recruit model outputs was investigated by comparing biological reference points obtained using growth curves derived for Umbrina robinsoni populations from False Bay, on the south coast of South Africa, sampled 10 years apart (1991–1993 and 2001–2003), and applying the two different age–length keys to length frequency data collected over the period 1991–1993 (n = 1 389) for the estimation of total mortality (Z), fishing mortality (F) and spawner biomass per recruit (SB/R). The SB/R and yield per-recruit (Y/R) curves constructed using biological data collected during the two different time periods were very similar, with target and threshold reference points differing only moderately (11%). This suggests that for long-lived species, the frequency of age and growth studies can be at least the lifespan of a cohort if they are to be assessed with a per-recruit approach. The use of the more recent length-at-age data, however, resulted in lower estimates of F (0.61 y?1 vs 1.05 y?1) and a slightly higher spawner biomass per-recruit ratio (19.5% vs 14.5%) for the earlier period. The reduced estimate of F when using the more recent dataset was likely due to a bias towards older fish caused by small sample size, the philopatric nature of U. robinsoni, and possible recruitment collapse. This highlights the importance of having a large, representative sample of length-at-age data for construction of age–length keys.  相似文献   

11.
During the past 30 years, 42 molecular studies have been undertaken in New Zealand to examine the phylogeography of coastal benthic invertebrates and plants. Here, we identify generalities and/or patterns that have emerged from this research and consider the processes implicated in generating genetic structure within populations. Studies have used various molecular markers and examined taxonomic groups with a range of life histories and dispersal strategies. Genetic disjunctions have been identified at multiple locations, with the most frequently observed division occurring between northern and southern populations at the top of the South Island. Although upwelling has been implicated as a cause of this disjunction, oceanographic evidence is lacking and alternative hypotheses exist. A significant negative correlation between larval duration and genetic differentiation (r2 = 0.39, P < 0.001, n = 29) across all studies suggests that larval duration might be used as a proxy for dispersal potential. However, among taxa with short larval durations (<10 days) there was greater variability in genetic differentiation than among taxa with longer pelagic periods. This variability implies that when larval duration is short, other factors may determine dispersal and connectivity among populations. Although there has been little congruence between the phylogeographic data and recognised biogeographic regions, recent research has resolved population subdivision at finer spatial scales corresponding more closely with existing biogeographic classifications. The use of fast‐evolving and ecologically significant molecular markers in hypothesis‐driven research could further improve our ability to detect population subdivision and identify the processes structuring marine ecosystems.  相似文献   

12.
In coastal populations of invertebrates and fishes, the distribution of discrete subpopulations is influenced by adult and larval dispersal, as well as by the effects of habitat heterogeneity on site fidelity or connectivity. Here, we examine evidence for spatial structure of sea perch, Helicolenus percoides, populations among four fjords in the Fiordland region of southwestern New Zealand. We examine patterns in adult morphology, length-at-age, δ13C and δ15N of muscle tissue, and trace elemental composition of whole otoliths as proxies for population isolation among the four inner fjord regions. A multivariate analysis of morphometrics reveals significant differences among populations from each of the four sites, suggesting existence of four distinct subpopulations. These patterns are consistent with observed differences in δ13C and δ15N, and length-at-age estimates among the four subpopulations. Differences in whole otolith concentrations of Sr, Ba, Mg and Li, and high classification scores based on the whole otolith elemental fingerprint are also consistent with significant subdivision among areas. Patterns across all four markers are consistent with discrete subpopulation structure of adult sea perch among the four study sites. These data indicate that the newly implemented network of marine protected areas in Fiordland is likely to contain discrete populations of sea perch.  相似文献   

13.
ABSTRACT

Increased temperatures and exposure to agricultural insecticides are increasingly threatening freshwater ecosystems worldwide. However, their combined effects are still poorly understood. We investigated the individual and interactive effects of water temperature and imidacloprid, the most widely used neonicotinoid insecticide, on larvae of two New Zealand mayflies, Deleatidium spp. and Coloburiscus humeralis. Ninety-six-hour bioassays with imidacloprid were performed at 9, 12, 15, 18, 21 and 24°C, in a full-factorial design. Mayfly survivorship and sub-lethal responses of impairment and moulting were recorded. We documented synergistically increased mayfly mortality and immobility with exposure to imidacloprid at higher temperatures, implying temperature-enhanced toxicity of imidacloprid. Survivorship of both mayflies was synergistically reduced by the combination of imidacloprid exposure and increasing temperatures. The same interaction also affected mobility of C. humeralis and moulting frequency of Deleatidium. Our findings suggest that developing a more thorough understanding of temperature-contaminant interactions may be important to allow better protection of aquatic ecosystems.  相似文献   

14.
The Pacific oyster Crassostrea gigas was introduced in Europe for commercial purposes in the mid 1960s. It was initially thought that low winter temperatures would restrain this species' reproduction and settlement; however, its present distribution in areas where no introduction has taken place suggests that natural invasion and expansion has occurred. Along the European coast, wild populations of Pacific oysters are already found from northern Germany to southern Portugal. Whether C. gigas will continue to further expand through northern waters will depend on its physiological performance. In this study, the performance of wild oyster populations has been studied in terms of growth and reproduction at three stations: La Rochelle (France; 46°N), Yerseke (Oosterschelde estuary, The Netherlands, 51°N), and Texel (Wadden Sea estuary, The Netherlands, 53°N). The French population had the lowest somatic-shell mass ratio and an increase in maximum shell length, somatic and gonadal mass was observed from France to the Netherlands. In addition, mean oocyte diameter decreased significantly from south to north. The combination of increasing gonadal mass and decreasing oocyte volume suggests an increasing reproductive output in terms of egg numbers from France to The Netherlands. Differences in temperature between locations will at least be partly responsible for the observed patterns; however, other environmental factors (such as food availability, predation pressure, sediment type and/or seston concentration) cannot be excluded. Since smaller eggs (oocytes) are thought to have a longer development time, the environmental conditions along the Dutch coast may result in increased larval dispersal and possibly in further population expansion.  相似文献   

15.
利用实验生态学方法,研究温度(15、19和23℃)、光照(明和暗)、仔鱼大小[全长(3.01±0.08)mm初孵仔鱼和(3.79±0.13)mm孵化后5天仔鱼]对稚海蜇[伞径(21.1±0.4)mm]捕食褐牙鲆仔鱼的影响。结果表明,稚海蜇对初孵仔鱼的捕食率随着水温升高而显著升高。光照条件在各仔鱼密度(10,30,50和80ind/L)下均不显著影响稚海蜇对初孵仔鱼的捕食率,说明光条件并不显著改善稚海蜇捕食仔鱼的能力。在高仔鱼密度(50和80ind/L)下,稚海蜇对孵化后5天仔鱼的捕食率显著低于对初孵仔鱼的捕食率;但在低密度(10和30ind/L)下,稚海蜇对两个不同发育阶段仔鱼的捕食率之间无显著差异。在低仔鱼密度下,稚海蜇对仔鱼的捕食率较低,这会降低仔鱼个体大小对稚海蜇捕食率作用的显著性。这既表明仔鱼逃避被稚海蜇捕食的能力随其发育生长而提高,也反映了稚海蜇对不同个体大小仔鱼捕食作用受仔鱼密度制约。  相似文献   

16.
A replicated mesocosm experiment was carried out to evaluate differential effects of feeding conditions for larval Northeast Arctic (NA) cod and Norwegian coastal (NC) cod. The two populations were (1) reared together with a 6-day older NA cohort (mixed) in high (HC) and low prey concentration (LC; 2000 and 200 prey/L initially), and (2) reared separately in HC treatments (non-mixed) to be able to evaluate both the effect of feeding conditions and possible effects of size interaction within mesocosms. The larvae were fed natural zooplankton, and the two stocks were identified in the mixed mesocosms by otolith marking. NA larvae hatched at a larger size, had higher growth rates, and survived better than NC larvae in both mixed and non-mixed mesocosms in the HC treatment. The second cohort clearly survived better in the non-mixed than in the mixed mesocosms, indicating the presence of an interaction effect before cannibalism could occur. We found a significant higher weight-at-length between NC and NA larvae (<12 mm), which was bigger than the effect difference due to feeding conditions. Furthermore, a positive relation between survival and initial growth within mesocosms was found. We suggest that lower growth at early larval stages was accompanied by lower survival, and suggest that this was further enhanced when larvae interacted with older and larger larvae.  相似文献   

17.
An enhanced version of the spatial ecosystem and population dynamics model SEAPODYM is presented to describe spatial dynamics of tuna and tuna-like species in the Pacific Ocean at monthly resolution over 1° grid-boxes. The simulations are driven by a bio-physical environment predicted from a coupled ocean physical–biogeochemical model. This new version of SEAPODYM includes expanded definitions of habitat indices, movements, and natural mortality based on empirical evidences. A thermal habitat of tuna species is derived from an individual heat budget model. The feeding habitat is computed according to the accessibility of tuna predator cohorts to different vertically migrating and non-migrating micronekton (mid-trophic) functional groups. The spawning habitat is based on temperature and the coincidence of spawning fish with presence or absence of predators and food for larvae. The successful larval recruitment is linked to spawning stock biomass. Larvae drift with currents, while immature and adult tuna can move of their own volition, in addition to being advected by currents. A food requirement index is computed to adjust locally the natural mortality of cohorts based on food demand and accessibility to available forage components. Together these mechanisms induce bottom-up and top-down effects, and intra- (i.e. between cohorts) and inter-species interactions. The model is now fully operational for running multi-species, multi-fisheries simulations, and the structure of the model allows a validation from multiple data sources. An application with two tuna species showing different biological characteristics, skipjack (Katsuwonus pelamis) and bigeye (Thunnus obesus), is presented to illustrate the capacity of the model to capture many important features of spatial dynamics of these two different tuna species in the Pacific Ocean. The actual validation is presented in a companion paper describing the approach to have a rigorous mathematical parameter optimization [Senina, I., Sibert, J., Lehodey, P., 2008. Parameter estimation for basin-scale ecosystem-linked population models of large pelagic predators: application to skipjack tuna. Progress in Oceanography]. Once this evaluation and parameterization is complete, it may be possible to use the model for management of tuna stocks in the context of climate and ecosystem variability, and to investigate potential changes due to anthropogenic activities including global warming and fisheries pressures and management scenarios.  相似文献   

18.
The sequential megafaunal collapse hypothesis: Testing with existing data   总被引：4，自引：1，他引：4  
Springer et al. [Springer, A.M., Estes, J.A., van Vliet, G.B., Williams, T.M., Doak, D.F., Danner, E.M., Forney, K.A., Pfister, B., 2003. Sequential megafaunal collapse in the North Pacific Ocean: an ongoing legacy of industrial whaling? Proceedings of the National Academy of Sciences 100 (21), 12,223–12,228] hypothesized that great whales were an important prey resource for killer whales, and that the removal of fin and sperm whales by commercial whaling in the region of the Bering Sea/Aleutian Islands (BSAI) in the late 1960s and 1970s led to cascading trophic interactions that caused the sequential decline of populations of harbor seal, northern fur seal, Steller sea lion and northern sea otter. This hypothesis, referred to as the Sequential Megafaunal Collapse (SMC), has stirred considerable interest because of its implication for ecosystem-based management. The SMC has the following assumptions: (1) fin whales and sperm whales were important as prey species in the Bering Sea; (2) the biomass of all large whale species (i.e., North Pacific right, fin, humpback, gray, sperm, minke and bowhead whales) was in decline in the Bering Sea in the 1960s and early 1970s; and (3) pinniped declines in the 1970s and 1980s were sequential. We concluded that the available data are not consistent with the first two assumptions of the SMC. Statistical tests of the timing of the declines do not support the assumption that pinniped declines were sequential. We propose two alternative hypotheses for the declines that are more consistent with the available data. While it is plausible, from energetic arguments, for predation by killer whales to have been an important factor in the declines of one or more of the three populations of pinnipeds and the sea otter population in the BSAI region over the last 30 years, we hypothesize that the declines in pinniped populations in the BSAI can best be understood by invoking a multiple factor hypothesis that includes both bottom–up forcing (as indicated by evidence of nutritional stress in the western Steller sea lion population) and top–down forcing (e.g., predation by killer whales, mortality incidental to commercial fishing, directed harvests). Our second hypothesis is a modification of the top–down forcing mechanism (i.e., killer whale predation on one or more of the pinniped populations and the sea otter population is mediated via the recovery of the eastern North Pacific population of the gray whale). We remain skeptical about the proposed link between commercial whaling on fin and sperm whales, which ended in the mid-1960s, and the observed decline of populations of northern fur seal, harbor seal, and Steller sea lion some 15 years later.  相似文献   

19.
The Jan Mayen area has an extreme environment with low temperatures and infrequent, but abrupt temperature changes. The shrimp population here is considered to be on its edge of distribution. The life-history parameters are in the same range as in other high-latitude shrimp populations and are characterized by slow growth, large size at maturation and extended longevity. Irregular and sporadic commercial exploitation limit fishing mortality and give the population life-history parameters not previously seen in other areas. The Jan Mayen shrimp are large compared to, e.g., the Barents Sea shrimp and can reach a maximum carapace length (Lmax) of 37 mm and an age of 10–11 years. The large size at sex transformation (L50, >24 mm) and analyses of length–frequency distributions indicate that the shrimp may be 6–7 years of age before changing sex. The change in Lmax and L50 observed during the study period is probably caused by increased natural mortality due to sudden temperature changes or due to increased predation, rather than increased growth rates. The life-history strategy of shrimp in the Jan Mayen area can be explained by factors such as depth, temperature and population density variations caused by fluctuation in recruitment and mortality.The shrimp fisheries in the Jan Mayen area began in the late 1970s and reached an annual landing of 2000 tonnes in 1985, and since then landings have oscillated around 500 tonnes depending on a combination of factors. The survey indices of stock biomass varied between 3000 and 6600 tonnes. For most years, the highest shrimp densities are at a depth of 200–299 m, while large shrimp (and therefore also female shrimp) are dominant at depths greater than 300 m.Fish community data were studied as the composition of the demersal fish community is an integrated response to environmental conditions and as predation affects the shrimp stock. Polar cod and capelin are the most abundant fish species in the study area. A high number of blue whiting was registered in 1979, but the number declined in 1980 and 1981 as temperature decreased. During the surveys in 1994 and 1995, no blue whiting was registered. A few individuals were found again in the 1999 samples. The number of Greenland halibut has declined from the beginning of the 1980s to the 1990s.  相似文献   

20.
Recent estimates of growth and mortality rates in extant Chesapeake Bay, USA oyster (Crassostrea virginica) populations are used to quantify changes in both population abundance (dN/dT) and shell accretion (dS/dT) associated with modern population demographics. The demographics of oyster populations that would be required to maintain reef accretion rates commensurate with sea level rise over geological time frames are examined using estimates of oyster longevity in pre-colonial (pre -1600) times combined with parallel estimates of pre-disease endemic mortality. The analysis demonstrates that modern populations, with their disease related, age-truncated demographics, are generally not capable of maintaining and building biogenic reefs through accretion. Estimates of filtration rates associated with Chesapeake Bay oyster populations prior to 1600 considerably underestimate actual benthic-pelagic coupling during that period. Pristine oyster populations would have supported water column turnover rates on the order of minutes to hours. Thus, the spatial footprint of oyster reefs was limited by available productivity in the estuary. Accretion rate calculations for pristine (pre-1600) oyster reefs describe the intimate relationship between benthic-pelagic coupling and the presence or absence of oyster reefs and the associated communities.  相似文献   

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