Paul G. Rodhouse

Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web

The Scotia Sea ecosystem is a major component of the circumpolar Southern Ocean system, where productivity and predator demand for prey are high. The eastward-flowing Antarctic Circumpolar Current (ACC) and waters from the Weddell–Scotia Confluence dominate the physics of the Scotia Sea, leading to a strong advective flow, intense eddy activity and mixing. There is also strong seasonality, manifest by the changing irradiance and sea ice cover, which leads to shorter summers in the south. Summer phytoplankton blooms, which at times can cover an area of more than 0.5 million km2, probably result from the mixing of micronutrients into surface waters through the flow of the ACC over the Scotia Arc. This production is consumed by a range of species including Antarctic krill, which are the major prey item of large seabird and marine mammal populations. The flow of the ACC is steered north by the Scotia Arc, pushing polar water to lower latitudes, carrying with it krill during spring and summer, which subsidize food webs around South Georgia and the northern Scotia Arc. There is also marked interannual variability in winter sea ice distribution and sea surface temperatures that is linked to southern hemisphere-scale climate processes such as the El Niño–Southern Oscillation. This variation affects regional primary and secondary production and influences biogeochemical cycles. It also affects krill population dynamics and dispersal, which in turn impacts higher trophic level predator foraging, breeding performance and population dynamics. The ecosystem has also been highly perturbed as a result of harvesting over the last two centuries and significant ecological changes have also occurred in response to rapid regional warming during the second half of the twentieth century. This combination of historical perturbation and rapid regional change highlights that the Scotia Sea ecosystem is likely to show significant change over the next two to three decades, which may result in major ecological shifts.

Cephalopod and Groundfish Landings: Evidence for Ecological Change in Global Fisheries?

Cephalopod fisheries are among the few still with some local potential for expansion; in fact, as groundfish landings have declined globally, cephalopod landings have increased. We propose the hypothesis that, although increased cephalopod landings may partly reflect increased market demand, overfishing groundfish stocks has positively affected cephalopod populations. Data from 15 key FAO areas reveal that, with the exception of the north- east Atlantic, cephalopod landings have increased significantly over the last 25 years while groundfish have risen more slowly, remained stable, or declined. In terms of volume, cephalopods have not replaced groundfish. This is hypothesized as owing to the shorter life cycle of cephalopods, and rapid turnover and lower standing stocks than for longer-lived finfish species. Under high fishing pressure, groundfish are probably poor competitors, having less opportunity for spawning and replacement. In West Africa, the Gulf of Thailand and Adriatic there is strong circumstantial evidence that fishing pressure has changed ecological conditions and cephalopod stocks have increased as predatory fish have declined. We recommend that this hypothesis be tested thoroughly in other areas where suitable data exist. Most coastal and shelf cephalopod fisheries are likely to be fully exploited or overexploited, and current annual fluctuations in cephalopod landings are probably largely environmentally-driven.

Food resource, gametogenesis and growth of Mytilus edulis on the shore and in suspended culture: Killary Harbour, Ireland

Mussels, Mytilus edulis L. grow on the shore and are cultured on ropes in Killary Harbour, a fjordic inlet on the Irish west coast. The food resource available to cultured mussels differs from that available to wild mussels on the shore. Although phytoplankton densities as estimated from chlorophyll a concentrations are similar, the shore environment in the inner part of the inlet is characterized by high mean POC concentrations. This is because of the presence of variable amounts of allochthonous detrital carbon. The annual cycles of flesh weight and ash content of wild and cultivated mussels were followed over two years. These cycles were related to the reproductive cycle observed by taking histological samples of mussel gonad, by plankton sampling for larvae and by monitoring larval settlement. Shell growth was measured in wild mussels by reading seasonal growth patterns on sectioned shells and in cultured mussels by following progress of the modal shell length of cohorts on ropes. Wild mussels have a partial spawning in early spring and spawn completely in the summer. Cultured mussels spawn twice during the summer, in the year following settlement. Growth rate of wild mussels decreases with increasing aerial exposure. The fastest growing mussels, at o % exposure, take about 6 years to attain the length attained by the mode of the cultured mussels after 18 months, when they are harvested. We conclude that wild mussels utilize a mix of phytoplankton and detritus as food during the summer and that large wild mussels can use detritus during the autumn and early winter for an increase in flesh weight and gametogenesis. This results in a partial spawning restricted to large individuals in the spring. Cultured mussels are mainly dependent on phytoplankton for food. This supports fast growth and two spawning bouts during the summer, but flesh weight declines once phytoplankton densities fall in the autumn.

Life cycles, oceanography and variability: ommastrephid squid in variable oceanographic environments

Abstract Exploited populations of ommastrephid squid are found under a wide range of oceanographic regimes. However, to date most scientific attention has focussed on those found in northern hemisphere western boundary current systems, and these systems have become the paradigm for theoretical work. Dosidicus gigas, in the east central-southeast Pacific, and Martialia hyadesi, in the southwest Atlantic, provide examples of the interactions between squid life cycles and regional oceanography outside of the paradigm. Illex argentinus, also in the southwest Atlantic, provides a southern hemisphere example of a western boundary current species. These examples are used to highlight the key issues involved in understanding squid population variability within the context of variable oceanographic environments. The issues include the fundamental influence of oceanographic variability on population variability, and the importance of a thorough understanding of the life cycle of the study species in order to detect and understand the relationships between it and the environment. The importance of understanding the relative temporal and spatial scales on which the environmental and biological factors interact is also considered. Overall, a broader understanding of the interactions between oceanographic variability and squid life cycles is necessary to interpret successfully the adaptation of ommastrephid squid species to their environment and to allow the effective forecasting and management of fishery resources.

Managing and forecasting squid fisheries in variable environments

Squid are short-lived ecological opportunists which generally have a lifespan of about 1 year. Their populations are labile and recruitment variability is driven, to a greater or lesser extent, by the environment. This variability provides a challenge to management because fisheries for short-lived species are best managed by effort limitation and it is difficult to set effort on a rational basis in the absence of information about the abundance of the next generation prior to recruitment. However, recent research has shown that recruitment variability in several squid species can be partly explained by environmental variability derived from synoptic oceanographic data. In the eastern Pacific coastal upwelling system a fishery for Dosidicus gigas has grown rapidly during the last decade and abundance and catch rates seem to be linked to the El Nino/southern oscillation (ENSO) cycle. ENSO is one of the best understood ocean/climate systems and so with increased knowledge of the life cycle biology of D. gigas, this fishery may provide a good model for understanding environmentally driven recruitment variability in exploited squid populations.

Southern ocean cephalopods

The Southern Ocean cephalopod fauna is distinctive, with high levels of endemism in the squid and particularly in the octopodids. Loliginid squid, sepiids and sepiolids are absent from the Southern Ocean, and all the squid are oceanic pelagic species. The octopodids dominate the neritic cephalopod fauna, with high levels of diversity, probably associated with niche separation. In common with temperate cephalopods, Southern Ocean species appear to be semelparous, but growth rates are probably lower and longevity greater than temperate counterparts. Compared with equivalent temperate species, eggs are generally large and fecundity low, with putative long development times. Reproduction may be seasonal in the squid but is extended in the octopodids. Cephalopods play an important role in the ecology of the Southern Ocean, linking the abundant mesopelagic fish and crustaceans with higher predators such as albatross, seals and whales. To date Southern Ocean cephalopods have not been commercially exploited, but there is potential for exploitation of muscular species of the Family Ommastrephidae.

Age, growth and population structure of the jumbo flying squid Dosidicus gigas in Peruvian waters

Age, growth and population structure of the jumbo flying squid, Dosidicus gigas, from the jig fishery in Peruvian waters in 1992 were determined by reading daily increments in ground and polished sections of statoliths. The squid ranged in size from 192 to 965 mm dorsal mantle length (ML) and no squid were older than 1 year. Two size groups were present in the exploited population; one group of small individuals 520 mm ML, with maximum ages of 220 and 354 days, respectively. The date of hatching estimated by back-calculation, revealed the presence of two cohorts of small squid; one hatched in autumn/winter and recruited to the fishery in spring/summer and the other hatched in spring/summer and recruited to the fishery in autumn/winter.

Cephalopods Occupy the Ecological Niche of Epipelagic Fish in the Antarctic Polar Frontal Zone

Recent data from research cruises and explorator fishing in the Antarctic Polar Frontal Zone (APFZ) of the Scotia Sea, together with data from dietary studies of Antarctic vertebrate predators, have revealed a large, previously overlooked trophic system in the Southern Ocean (Fig. 1). The upper trophic levels of this open-ocean epipelagic community are exceptional in that they contain no fish species. Fishes are replaced by cephalopods, including the ommastrephid squid, Martialia hyadesi. This squid preys on mesopelagic m.yctophids (lanternfish), which feed largely on copepods. We identify here a geographically distinct, Antarctic, open-ocean food chain which is of importance to air breathing predator species but where Antarctic krill, Euphausia superba, is absent. This system is probably prevalent in areas of higher primary productivity, especially the Scotia Sea and near the peri-Antarctic islands. Squid stocks in the APFZ may have potential for commercial exploitation, but they, and the predators they support, are likely to be sensitive to overfishing. Squid have a short, semelparous lifecycle, so overfishing in a single year can cause a stock to collapse.

Cephalopod prey of the wandering albatross Diomedea exulans

Cephalopod beaks from the stomach contents of “wandering albatross” (Diomedea exulans L.) chicks from Bird Island, South Georgia, were sampled between May and September in 1983 and 1984. Lower beaks were identified and measured, and allometric data were used to calculated mantle length and biomass of the species consumed. A total of 3421 lower beaks were examined, representing 35 species in the 1983 sample and 45 species in the 1984 sample. Eight of the twenty families contributed over 95% of the biomass. In 1984 there were less Onychoteuthidae and more Ommastrephidae than in 1983 and a decrease in the number of species known to occur south of the Antarctic Polar Front. There was a difference in the size-frequency distribution of the cephalopod diet in the two years; in 1984 there was a higher frequency of intermediate-sized specimens, reflecting the greater importance of ommastrephids, especially Illex sp. The energy content of cephalopods in 1984 may have been greater than in 1983. Serial sampling of cephalopod beaks during the austral winter did not reveal evidence of growth. By the age of 200 d, wandering albatross chicks have consumed a total of approximately 100 kg wet weight of cephalopods each.

Remote sensing of the global light-fishing fleet: an analysis of interactions with oceanography, other fisheries and predators

Publisher Summary The use of United States Defense Meteorological Satellite Program Operational Linescan System (DMSP-OLS) data has enabled the precise location of the distribution of the global light-fishing fleet over a six month period in relation to the large and general mesoscale oceanography of the ecological provinces where they occur. The squid catch in these light fisheries can be identified to genera or species in seven ecological provinces where DMSP-OLS imagery reveals light-fishing activities. The DMSP-OLS data provide the information needed to review the relationship between the squid fisheries using lights and other fisheries for finfish with better spatial resolution than has been previously possible using data for FAO statistical areas alone. This chapter demonstrates that 62–70%, and possibly up to something