Ontogenetic loops in habitat use highlight the importance of littoral habitats for early life-stages of oceanic fishes in temperate waters

General concepts of larval fish ecology in temperate oceans predominantly associate dispersal and survival to exogenous mechanisms such as passive drift along ocean currents. However, for tropical reef fish larvae and species in inland freshwater systems behavioural aspects of habitat selection are evidently important components of dispersal. This study is focused on larval Atlantic herring (Clupea harengus) distribution in a Baltic Sea retention area, free of lunar tides and directed current regimes, considered as a natural mesocosm. A Lorenz curve originally applied in socio-economics to describe demographic income distribution was adapted to a 20 year time-series of weekly larval herring distribution, revealing size-dependent spatial homogeneity. Additional quantitative sampling of distinct larval development stages across pelagic and littoral areas uncovered a loop in habitat use during larval ontogeny, revealing a key role of shallow littoral waters. With increasing rates of coastal change, our findings emphasize the importance of the littoral zone when considering reproduction of pelagic, ocean-going fish species; highlighting a need for more sensitive management of regional coastal zones

Effect of sampling effort and sampling frequency on the composition of the planktonic crustacean assemblage: a case study of the river Danube

Although numerous studies have focused on the seasonal dynamics of riverine zooplankton, little is known about its short-term variation. In order to examine the effects of sampling frequency and sampling effort, microcrustacean samples were collected at daily intervals between 13 June and 21 July of 2007 in a parapotamal side arm of the river Danube, Hungary. Samples were also taken at biweekly intervals from November 2006 to May 2008. After presenting the community dynamics, the effect of sampling effort was evaluated with two different methods; the minimal sample size was also estimated. We introduced a single index (potential dynamic information loss; to determine the potential loss of information when sampling frequency is reduced. The formula was calculated for the total abundance, densities of the dominant taxa, adult/larva ratios of copepods and for two different diversity measures. Results suggest that abundances may experience notable fluctuations even within 1 week, as do diversities and adult/larva ratios

Growth patterns of the pearl oyster Pinctada margaritifera L. in Gazi Bay, Kenya

Culture of pearl oysters is rapidly increasing worldwide, including the western Indian Ocean. The oyster Pinctada margaritifera L., which produces the most highly valued black pearls, occurs in East Africa, and has been exploited there for the shell for many decades. The growth patterns of P. margaritifera from a natural population in the sheltered back-reef, and from oysters translocated to a tidal current-swept site, both sites within Gazi Bay, Kenya, are described. The growth rate in the natural population ranged from 31.3mm year–1 (60–65mm size-class) to 7.6mm year–1 (105–110mm sizeclass). The von Bertalanffy growth calculated with a fixed L&#8734 of 127.2mm, was 0.30 for the natural population and 0.38 for the translocated oysters. The mean growth rate during the north-east monsoon season was approximately double that for during the south-east monsoon season. The daily rate of nacre deposition ranged from 1.3µm to 5.9µm (mean 3.45µm); it declined with the size of oysters and was marginally higher at the high-energy current site. At that rate, it would take approximately two years to produce a marketable cultured half pearl with a 2.5mm layer of nacre. The results of the study are relevant to the understanding of the influence of the environment on growth, and are applicable to the optimisation of growth rate of pearl oysters in the inshore region along the east coast of Africa. Keywords: growth rate, Kenya, monsoon seasons, nacre deposition, Pinctada margaritifera, tidal currentsAfrican Journal of Marine Science 2005, 27(3): 567–57

Size at onset of maturity of spiny lobsters Panulirus homarus homarus at Mambrui, Kenya

The size at onset of sexual maturity of the scalloped spiny lobster Panulirus homarus homarus captured off Mambrui, Kenya, was estimated from ovary condition, presence of external eggs and/or spermatophore remains (berry method), and presence of well developed ovigerous setae (setal method). The predictive utility of morphometric relationships between carapace length (CL), pleopodal exopodite length (exopod length) and second leg length were assessed by fitting linear regressions to lobsters categorised as immature and mature, and comparing the intersect values with other estimates of functional maturity. The smallest female with mature ovaries was 52mm CL, compared with the smallest specimens of 50mm with ovigerous setae and 54mm with external eggs. Logistic curves indicated that 50% maturity was reached at 52.6mm (setal method) or 63.4mm (berry method) during the main breeding season. Female exopod lengths suggested that functional maturity was reached at 50.5mm and male leg lengths increased significantly upon reaching 57.5mm. The analyses indicate that measurements of female exopods and male second leg length can be used as cost-effective methods to estimate size at functional maturity of P. homarus homarus in Kenyan waters, and that these indices can augment estimates based on the presence of external eggs and ovigerous setae.Keywords: allometry, Kenya, Panulirus homarus, sexual maturityAfrican Journal of Marine Science 2006, 28(1): 51–5

Zooplankton of Lake Kivu

peer reviewedThe dominant species of the crustacean plankton in Lake Kivu are the cyclopoid copepods Thermocyclops consimilis and Mesocyclops aequatorialis and the cladoceran Diaphanosoma excisum. Mean crustacean biomass over the period 2003–2004 was 0.99 g C m−2. The seasonal dynamics closely followed variations of chlorophyll a concentration and responded well to the dry season phytoplankton peak. The mean annual crustacean production rate was 23 g C m−2 year−1. The mean trophic transfer efficiency between phytoplankton and herbivorous zooplankton was equal to 6.8 %, indicating a coupling between both trophic levels similar to that in other East African Great lakes. These observations suggest a predominant bottom-up control of plankton dynamics and biomass in Lake Kivu. Whereas the present biomass of crustacean plankton in Lake Kivu is comparable to that of other African Rift lakes, the zooplankton biomass before Limnothrissa introduction was 2.6 g C m−2, based on estimation from available historical data. So, if the sardine introduction in the middle of the last century led to a threefold decrease of zooplankton biomass, it did not affect zooplankton production to a level which would lead to the collapse of the food web and of the fishery