The link between migration, the reproductive cycle and condition of Sardinella aurita off Mauritania, north-west Africa

The annual migration pattern of round sardinella Sardinella aurita up and down the north-western African coast between 12° N (Senegal) and 22° N (western Sahara) was shown to be associated with spawning activity and a distinct seasonality in fish condition, based on monthly sampling from commercial catches (2000–2003). Some S. aurita were found to spawn throughout the entire year, but a peak in spawning existed during the summer (June to September). The spawning cycle is apparent from seasonality in maturity stages, but is also demonstrated by the increase in gonad mass and fat content of the fish in springtime, the period preceding spawning. During the months after spawning, although feeding is maintained, the physical condition of the fish collapses, and fat content rapidly declines.     

Gape morphology of cod Gadus morhua L., haddock Melanogrammus aeglefinus (L.) and whiting Merlangius merlangus (L.) through metamorphosis from larvae to juveniles in the western Irish Sea

Variations in standard length ( L _S), gape size ( S _G) and jaw length ( L _J) were studied in larval and juvenile gadoids (cod Gadus morhua , haddock Melanogrammus aeglefinus and whiting Merlangius merlangus ) from 4 to 70 mm. The increase in S _G and L _J was not linear with respect to L _S. The relationship was best described by segmented regression lines in all three species, with an inflection point at c . 10·5 mm. The S _G and L _J increased more rapidly in relation to larval L _S for individuals smaller than this inflection point size. The rates of increase slowed significantly post-inflection, an effect more noticeable in S _G data compared to L _J data. In each case, the inflection point fell in the intermediate period of development between the larval and juvenile stages, which could be considered as metamorphosis. Published equations that have been used to predict S _G from L _J lead to the overestimation of gape. New relationships are presented, which may be used to predict S _G from measurements of either L _S or upper jaw length in cod, haddock and whiting.     

The role of thermal stratification on the population dynamics of Sagitta elegans Verrill in the western Irish Sea

Temperature and prey availability have a marked effect on thedevelopment and growth of Sagitta elegans. This study addressesthe effects of thermal stratification on the growth and sizeof adult S.elegans in the western Irish Sea. It shows that instratified waters, bottom water temperature had a greater effecton the development of mature adults than surface water temperature.Whilst the length of mature individuals caught in 1992 and 1994decreased with increasing temperature, they did not conformto the models of McLaren (1963) and Sameoto (1971), as the animalswere larger than predicted. The long breeding period found inthe stratified region, suggests that previous reports of oneor two generations per year in the western Irish Sea do notaccount for the likely mix of generations that occur from Februarythrough to September.     

Copepod abundance in the western Irish Sea: relationship to physical regime, phytoplankton production and standing stock

The distinct patterns of stratification in the North Channeland stratified region of the western Irish Sea influence theseasonal abundance of phytoplankton. The 3–4 month productionseason in the stratified region was characterized by productionand biomass peaks in the spring (up to 2378 mg C m² day^–1and 178.4 mg chlorophyll m^–2) and autumn (up to 1280 mgC m^–2 day^–1 and 101.9 mg chlorophyll m^–2).Phytoplankton in the North Channel exhibited a short, late productionseason with a single summer (June/July) peak in production (4483mg Cm^–2 day^–1) and biomass (–160.6 mg chlorophyllm^–2). These differences have little influence on copepoddynamics. Both regions supported recurrent annual cycles ofcopepod abundance with similar seasonal maxima (182.8–241.810³ind. m^–2) and dominant species (Pseudocalanus elongatusand Acartia clausi). Specific rates of population increase inthe spring were 0.071 and 0.048 day¹ for the North Channel andstratified region, respectively. Increased copepod abundancein the stratified region coincided with the spring bloom, andwas significantly correlated with chlorophyll standing stock.Increased copepod abundance preceded the summer production peakin the North Channel. This increase was not correlated withchlorophyll standing crop, suggesting that a food resource otherthan phytoplankton may be responsible for the onset of copepodproduction prior to the spring bloom. Hetero-trophic microplanktonas an alternative food source, and advection of copepods fromthe stratified region, are proposed as possible explanationsfor copepod abundance increasing in advance of the summer peakin primary production.     

Descriptions of the Gulf VH/PRO-NET and MAFF/Guildline unencased high-speed plankton samplers

Two high-speed plankton samplers (Gulf VII/PRO-NET and MAFF/Guildline)currently in use to sample pelagic eggs and larvae in watersaround the UK are described. Both samplers are modificationsof Gulf III type samplers and are capable of being towed atspeeds of up to 7 knots (3.6 m s^–1). The performance ofthe samplers (flow rates, degree of clogging of the nets andsample depth profiles) can be monitored during deployment. Thereal-time monitoring packages also provide vertical profilesof temperature, salinity and chlorophyll a fluorescence. Bothsamplers can be constructed in different sizes to suit the vesselfrom which the sampler is deployed and the expected concentrationof plankton in the sampling area.     

Temperature-dependent stage durations of Irish Sea Nephrops larvae

The relationships between temperature and stage duration ofthe three zoea stages of Irish Sea Nephrops norvegicus wereinvestigated by culturing individual larvae at five differenttemperatures. The relationships are required for estimatingdaily larval production from survey data on abundance. The survivalof cultured larvae was relatively high. Estimates of the temperature–stageduration relationships from data of individual larvae were differentfrom a previous Irish Sea study. However, using the averageduration at each temperature gave relationships that were notsignificantly different from previous studies in the Irish Seaand Clyde. No larvae were successfully reared to stage IV (post-zoea)at lower temperatures, so it was impossible to assess the relationshipbetween stage III larval duration and temperature.     

Mapping the spawning grounds of North Sea cod (Gadus morhua) by direct and indirect means

Despite recent evidence for sub-stock structuring, North Sea cod are assessed as a single unit. As a consequence, knowledge of sub-stock trends is poor. In particular, there are no recent evaluations of which spawning grounds are active. Here we report results from the first ichthyoplankton survey to cover the whole North Sea. Also, this survey, conducted in 2004, was the first to make extensive use of DNA-based molecular methods to unambiguously identify early developmental stage cod eggs. We compare the findings from the plankton survey with estimated egg production inferred from the distribution of mature cod in contemporaneous trawl surveys. Results from both approaches were in general agreement and showed hot spots of egg production around the southern and eastern edges of the Dogger Bank, in the German Bight, the Moray Firth and to the east of the Shetlands. These areas broadly coincide with known spawning locations from the period 1940 to 1970. We were, however, unable to directly detect significant numbers of cod eggs at the historic spawning ground off Flamborough (northeast coast of England). The results demonstrate that most of the major spawning grounds of cod in the North Sea are still active but that some localized populations may have been reduced to the point where it is now difficult to detect the presence of eggs in the plankton.     

The occurrence of Calanus finmarchicus (Gunnerus) and Calanus helgolandicus (Claus) in the western Irish Sea

The seasonal abundance of Calanus finmarchicus and Calanus helgolandicusin the North Channel and stratified region of the western IrishSea is reviewed using data collected between 1992 and 1996.Both species occur in the western Irish Sea, but were more abundantin the stratified region during spring. Increased abundanceduring May/June was attributed to an increase in copepoditestages. Calanus helgolandicus dominated in both regions, exhibitingspring and autumn peaks in abundance in the stratified region.It is argued that the presence of ripe females and naupliarstages in the stratified region is evidence of an in situ breedingpopulation, rather than advection of individuals from populationcentres outside the Irish Sea. The lack of geographical separationof the two species in the western Irish sea, and reports thatboth species occur in the Celtic Sea and Malin Shelf, limitthe use of either species as indicators of exchange processesbetween the Irish Sea and neighbouring waters.     

The occurrence of juvenile pigmentation abnormalities in plaice (Pleuronectes platessa) larvae fed on enriched and unenriched Artemia salina nauplii

The effect of diet on the frequency of malpigmentation was investigated in laboratory-reared plaice ( Pleuronectes platessa ) fed on either enriched or unenriched Artemia salina nauplii . The juveniles fed continuously on unenriched Artemia during their larval period showed 29.0% normal pigmentation whereas those fed solely on enriched Artemia , showed 63% normal pigmentation. The possibility of a critical period for pigment development was tested by changing the diet of larvae at different ages. Results suggested that a critical period is less important than the duration of time fed on a nutritionally complete diet.     

Does the western Irish Sea gyre influence the distribution of pelagic juvenile fish?

The western Irish Sea is an important spawning and nursery ground for many commercially exploited fish. Spawning begins in the Irish coastal regions in early spring, and moves offshore as the season develops. As a result of the onset of seasonal heating in spring, stratification isolates a dome of cold bottom winter water in the deep (>100 m) basin of the western Irish Sea. The resultant density fields drive a cyclonic gyre which dominates the circulation of the region during late spring and summer and is characterized by anticlockwise current speeds which exceed 0.2 ms^–1, after removal of tides. Surveys of pelagic juvenile (0-group) fish in 1994, 1995 and 1996, showed that they were coincident with the centre of the gyre. Physical data from 1994 and 1995, were used to describe the horizontal and vertical structure of the water column and the associated circulation regime. The behaviour and swimming speeds of pelagic juvenile fish were insufficient to explain their apparent shift in abundance away from the coast to the central western Irish Sea. Drifter and current data suggested that their entrainment into the gyre could provide both the transport and retention mechanisms.