Neil L Andrew

TROPHIC STRUCTURE OF COASTAL ANTARCTIC FOOD WEBS ASSOCIATED WITH CHANGES IN SEA ICE AND FOOD SUPPLY

Predicting the dynamics of ecosystems requires an understanding of how trophic interactions respond to environmental change. In Antarctic marine ecosystems, food web dynamics are inextricably linked to sea ice conditions that affect the nature and magnitude of primary food sources available to higher trophic levels. Recent attention on the changing sea ice conditions in polar seas highlights the need to better understand how marine food webs respond to changes in such broad-scale environmental drivers. This study investigated the importance of sea ice and advected primary food sources to the structure of benthic food webs in coastal Antarctica. We compared the isotopic composition of several seafloor taxa (including primary producers and invertebrates with a variety of feeding modes) that are widely distributed in the Antarctic. We assessed shifts in the trophic role of numerically dominant benthic omnivores at five coastal Ross Sea locations. These locations vary in primary productivity and food availability, due to their different levels of sea ice cover, and proximity to polynyas and advected primary production. The delta15N signatures and isotope mixing model results for the bivalves Laternula elliptica and Adamussium colbecki and the urchin Sterechinus neumeyeri indicate a shift from consumption of a higher proportion of detritus at locations with more permanent sea ice in the south to more freshly produced algal material associated with proximity to ice-free water in the north and east. The detrital pathways utilized by many benthic species may act to dampen the impacts of large seasonal fluctuations in the availability of primary production. The limiting relationship between sea ice distribution and in situ primary productivity emphasizes the role of connectivity and spatial subsidies of organic matter in fueling the food web. Our results begin to provide a basis for predicting how benthic ecosystems will respond to changes in sea ice persistence and extent along environmental gradients in the high Antarctic.

Accounting for local scale variability in benthos: implications for future assessments of latitudinal trends in the coastal Ross Sea

This paper is a contribution to the Latitudinal Gradient Project. It describes macro and epifaunal assemblages and habitats at three shallow water locations at the southern end of the western Ross Sea coast, and investigates relationships between faunal composition and environmental characteristics. Many variables (e.g. substrate type, sediment composition, depth, latitude, longitude) contributed to explaining the differences in community composition between locations, with latitude (a likely surrogate for broader scale factors, e.g. ice cover) one of the most important. The percentage explained by environmental characteristics was strongly scale dependent, decreasing with increasing scale of observation. As much as 66% and 75% of the variability in macrofaunal and epifaunal assemblages, respectively, was explained at the smallest scale (i.e. between transects within a location), compared to 9–18% and 11–32%, respectively, at the scale of the entire study. This relationship was also true for species richness and total abundance. This suggests that while small-scale habitat variability will not confound our ability to detect latitudinal gradients in future studies, adequately quantifying the environmental factors important in structuring these communities at larger (latitudinal) spatial scales will be important. Finally, large differences in habitat structure did not translate into large differences in the diversity of fauna, illustrating the difficulty of predicting faunal composition in the Ross Sea based on seafloor topography alone.

A length-based Bayesian stock assessment model for the New Zealand abalone Haliotis iris

We describe a length-based Bayesian model for stock assessment of the New Zealand abalone Haliotis iris (paua). We fitted the model to five data sets: catch-per-unit-effort (CPUE) and a fishery-independent survey index, proportions-at-length from both commercial catch sampling and population surveys, and tag–recapture data. We estimated a common component of error and used iterative re-weighting of the data sets to balance the residuals, removing the arbitrary data set weightings used in previous assessments. Estimates at the mode of the joint posterior distribution were used to explore sensitivity of the results to model assumptions and input data; the assessment itself was based on marginal posterior distributions estimated from Markov chain–Monte Carlo simulation. Assessments are presented for two stocks in the south of New Zealand. One may be recovering after recent catch reductions; the other is over-exploited and likely to decline further. Assessment for the first stock was robust; assessment for the second stock was sensitive to the CPUE data and may be too optimistic. We discuss future directions and potential problems with this approach.

Growth and reproductive phenology of the kelp Lessonia variegata in central New Zealand

Abstract A field study of Lessonia variegata growing in depths of 8–10 m on the south coast of Wellington, New Zealand investigated morphometric and biomass indices measured on destructively harvested entire algae, rates of blade elongation measured in situ using a blade hole punch technique, and timing of fertility over a 2‐year period. Blade fertility was estimated using a fertility score based on the visual appearance of the reproductive tissue. Lessonia variegata attained a maximum length of 154 cm, a density of 0.8 to 2.1 individuals m‐2, and developed an areal biomass ranging from 1700 to 6000 g fresh weight (FW) m‐2. The highest proportion of fertile blades (up to 100% of sampled blades) occurred during winter and the end of the fertile period coincided with the onset of the longer days and warmer temperatures of summer. Blade elongation occurred more rapidly in summer months (0.117cm day‐1, SE = 0.015) than winter months (0.041cm day‐1, SE = 0.009). The slower winter blade elongation rate meant that growth could not overcome tip ablation and average blade length reduced from 51.5cm (SE = 1.9) to 30.8 cm (SE = 1.7) between March and September 2003. Average blade length increased again over the following summer months to 40.7cm (SE = 1.6). Blade biomass dominated total fresh weight biomass (average 64% of total). Two parameters that are relatively easily measured on live macroalgae in situ, holdfast diameter and girth of all stipes collectively, predicted total biomass with an r2 = 0.91, suggesting these could be useful tools for assessment of this potentially harvestable resource on Wellingtons south coast.

Stock Enhancement Initiatives

Publisher Summary This chapter reviews stock enhancement initiatives for a wide variety of marine invertebrates such as, Scallops, Other Bivalves, Abalone, Queen Conch, Shrimp, Spiny Lobsters, Lobsters, and Sea Urchins. Stock enhancement of scallops stands out among marine invertebrates for the nature and scale of the operations. A major advantage of using wild spat for stock enhancement of scallops is that the seed has no inherent morphological, behavioral or genetic deficits. Another major problem limiting the potential for stock enhancement of queen conch is the limited availability of nursery habitats. A limiting factor for restocking and stock enhancement programs for Abalone may be the inability of hatcheries to produce the required number of juveniles. The potential for extensive mariculture of Queen Conch was considered to be low because of high hatchery costs, lack of mass-rearing techniques, and high predation on juveniles. The great economic value of Shrimp fisheries has led to very high exploitation and prompted several countries to assess the potential for stock enhancement programs. Two main types of interventions are underway to increase production of Spiny Lobsters further. Major problem limiting the potential for stock enhancement of Queen Conch is the limited availability of nursery habitats. Much of the basic technical and ecological knowledge required to mass produce juvenile Sea Urchins and release them with a relatively high probability of survival is now in place.

Other Important Considerations for All Initiatives

Publisher Summary This chapter presents the differences between restocking and stock enhancement, several considerations apply equally to both types of interventions, where they are deemed to be beneficial to the management of invertebrate fisheries. These considerations are the need to measure whether the intervention has been as successful as expected, minimize the potential negative impacts of releasing cultured juveniles on the genetic integrity of the wild populations, reduce the risk of introducing diseases to the target species and broader animal community, and avoid undesirable changes to the relative abundances of other valuable species in the ecosystem. The changes to the genetic diversity of target stocks, and the risks of introducing diseases to conspecifics and other species in the ecosystem, are not the only potential impacts of restocking and stock enhancement. There is concern that the changes in relative abundance of target species, brought about by successful large-scale releases or associated manipulations, may be at the expense of other valuable components in the ecosystem. Another feature of the stock enhancement of marine invertebrates to be considered when assessing potential impacts of releases is that many of the target species feed low in the food chain.

Overview and Progress Towards a Responsible Approach

Publisher Summary This chapter presents the approaches and outcomes across species and outlines general progress toward the components of a responsible approach done separately for the species/groups used in restocking and those used for stock enhancement initiatives. A key feature of the restocking initiatives for marine invertebrates described in the chapter is that the cultured juveniles have always been produced in hatcheries. Two striking features of the restocking initiatives, compared with stock enhancement programs, are that they are all at an early stage and that they are underway mainly in developing countries and Small Island Developing States (SIDS) with the assistance of international partners. In contrast to restocking initiatives, several different methods have been used to supply juveniles for marine invertebrate stock enhancement programs, these include collection and nursing of wild spat, translocation of excess juveniles that have settled naturally, hatchery-reared juveniles, and provision of suitable habitat to enable spat to settle in greater numbers or in areas where settlement would not otherwise be possible. The chapter discusses the stock enhancement initiatives for the eight species/groups that provide more opportunities to assess progress toward a responsible approach than the work done so far on restocking. It is evident that capture and culture of wild spat is one of the simplest and most attractive options for stock enhancement; it reduces the cost of supplying large numbers of environmentally fit juveniles and can avoid effects of releases on the genetic structure of stocks.

Management of Restocking and Stock Enhancement Programmes

Publisher Summary This chapter outlines the basic information required to determine whether restocking is likely to help rebuild severely depleted stocks or whether stock enhancement could help overcome recruitment limitation in operational fisheries and return a profit on investment. The chapter demonstrates that either restocking or stock enhancement is likely to be beneficial and identifies various ways in which the released animals should be managed to obtain optimum benefits from these two types of interventions. It explains whether release of cultured juveniles will add value to other measures normally used to rebuild spawning biomass to more productive levels. Managers need sound information on stock delineation, that is, the biological and genetic structure of the population(s) comprising the fishery, the projected time frames for recovery with and without various levels of restocking, and the capacity of hatcheries to produce juveniles. The managers of restocking programs are aiming for long-term benefits, and the managers of stock enhancement programs need to demonstrate, often over a short term, that the value of the additional harvest exceeds the cost of producing and releasing the juveniles. The chapter focuses on development of regulations to optimize the biological, social, and financial sustainability of the interventions made to increase the productivity of the fishery and need to pay particular attention to the number of juveniles released and to identifying and implementing measures that maximize the benefits from the process.