Water temperature measured in Independence Bay, Pisco, Peru

Independencia Bay can be considered as one of the most productive invertebratc fishing grounds worldwide. One of the most important exploited species is the scallop (Argopecten purpuratus) with strong catching fluctuations related to El Nino and La Nina events and to inadequate Management strategies. During strong warming periods annual landings reach up to 50000 t in an area of about 150 km**2 and during cold years they remain around 500 to 1000 t. This study analyses the changes in scallop landings at Independencia Bay observed during the last two decades and discusses the main factors affecting the scallop proliferations during the EI Nino events. In this way data on landings, sea surface temperature and those related to growth, reproduction, predation, mean density and oxygen concentration from published and unpublished Papers are used. The relationship between annual catches and average water temperature over the preceding reproductive period of the scallop over the past 20 year's period, showed that scallop production is affected positively only with strong EI Nino such as those of 1983 and 1998. Our review showed that the scallop stock proliferation can be traced to the combined effect of (1) an increase in reproductive output through an acceleration of gonad maturation and a higher spawning frequency; (2) a shortening of the larval period and an increase in larval survival; (3) an increase in the individual growth performance; (4) an increase in the juvenile and adult survival through reduction of predator biomass; (5) an increase in carrying capacity of the scallop banks due to elevated oxygen levels.

Abundance of copepods from multinet samples during POLARSTERN cruise ANT-XXII/2 (ISPOL)

The mesozooplankton community, with special emphasis on calanoid copepods, was studied with respect to its species composition, abundance, vertical distribution and developmental structure during the ISPOL expedition to the ice covered western Weddell Sea. Stratified zooplankton tows were carried out nine times between December 1, 2004 and January 2, 2005 with a multiple opening-closing net between 0 and 1000 m depth. Copepods were by far the most abundant taxon contributing more than 94% of the total mesozooplankton. Numerical dominants were cyclopoid copepods, mostly Oncaea spp. A total of 66 calanoid copepod species were identified, but the calanoid copepod community was characterised by the dominance of only a few species. The most numerous species was Microcalanus pygmaeus, which comprised on average 70% of all calanoids. Calanoides acutus and Metridia gerlachei represented other abundant calanoid species contributing an average of 8 and 7%, respectively. All other species comprised less than 3%. The temporal changes in the abundance and population structure of M. pygmaeus and M. gerlachei were small while a shift in the stage frequency distribution of C. acutus was observed during the study: CIV dominated the C. acutus population with 48 to 50% during the first week of December, while CV comprised 48% in late December. CI and CII of C. acutus were absent in the samples and males occurred only in very low numbers in greater depths. In M. gerlachei, CI was not found, whereas all developmental stages of M. pygmaeus occurred throughout the study. All three species showed migratory behaviour, and they occurred in upper water layers towards the end of the investigation. This vertical ascent was most pronounced in C. acutus and relatively weak in the other two species. In M. pygmaeus and M. gerlachei, copepodite stages were responsible for the upward migration in late December, while the vertical distribution of adults did not change. In C. acutus all abundant developmental stages (CIV, CV and females) ascended to upper water layers. Almost exclusively (93%) medium- and semi-ripe females of C. acutus and M. gerlachei were found, and only 3 - 4% of the ovaries were ripe. The absence of CI and the low number of ripe females indicate that the main reproductive period had not started in C. acutus and M. gerlachei until the end of our study in early January. In contrast, the high portion of CI and CII of M. pygmaeus suggests that reproduction of this species had started in October-November and hence, before the onset of the phytoplankton bloom in the water. The community structure did not differ between stations with one exception on December 26, when the station was strongly influenced by the continental shelf.

(Table 1) Mean sizes of Hydrobia ulvae juveniles in the littoral zone of the Chernaya Inlet (Kandalaksha Bay, White Sea) in July-September 1995

Based on observations and experiments carried out within the White Sea silty-sandy littoral zone in 1994-1997 data on biology of development and behavior of Hydrobia ulvae juveniles over water column and in sediments were obtained. Hydrobiid juveniles 0.125-0.150 mm in size appear in plankton during the second half of June and in two to three weeks they precipitate on sediments reaching 0.300-0.350 mm in size. Specific biological features of the White Sea hydrobiids are a short reproductive period and a short period of juvenile growth related to long under-ice time and decelerated warming of shallow waters. Distribution of juvenile individuals of H. ulvae is primarily determined by hydrodynamics and microtopography of the littoral zone. Redistribution of the juveniles permanently takes place, since all size groups of the juveniles are equally subjected to migration. During the first few weeks after settling mortality of juvenile mudsnails is 85%.

Barnacle recurit density, biomass, and reproductive output measured in rocky intertidal habitats in Glasgow Head, on the Atlantic coast of Nova Scotia, Canada

Through a field experiment, we show that a predator has negative nonconsumptive effects (NCEs) on different life-history stages of the same prey species. Shortly before the recruitment season of the barnacle Semibalanus balanoides (May-June), we established experimental cages in rocky intertidal habitats in Nova Scotia, Canada. The cages were used to manipulate the presence and absence of dogwhelks, Nucella lapillus, the main predators of barnacles. At the centre of each cage, we installed a tile where barnacle pelagic larvae could settle and the resulting recruits grow. Mesh prevented caged dogwhelks from accessing the tiles, but allowed waterborne dogwhelk cues to reach the tiles. Results in May indicated that barnacle larvae settled preferentially on tiles from cages without dogwhelks. In November, at the end of the dogwhelk activity period and once the barnacle recruits had grown to adult size, barnacle body mass was lower in the presence of dogwhelks. This limitation may have resulted from a lower barnacle feeding activity with nearby dogwhelks, as found by a previous study. The observed larval and adult responses in barnacles are consistent with attempts to decrease predation risk. November data also indicated that dogwhelk cues limited barnacle reproductive output, a possible consequence of the limited growth of barnacles. Overall, this study suggests that a predator species might influence trait evolution in a prey species through NCEs on different life-history stages.