Gorka Sancho

PREDATION STRUCTURES COMMUNITIES AT DEEP-SEA HYDROTHERMAL VENTS

The structure and dynamics of natural communities result from the interplay of abiotic and biotic factors. We used manipulative field experiments to determine the relative roles of abiotic conditions and biotic interactions in structuring deep-sea (2500 m depth) communities along environmental gradients around hydrothermal vents of the eastern tropical Pacific Ocean (East Pacific Rise, at 9 50 N). We tested (1) whether predation by crabs and fishes affects the recruitment of benthic species and subsequent community structure and (2) whether the effects of predation vary along the steep gradients of tem- perature, oxygen, sulfide, and metal concentrations near vents. Recruitment substrates (ba- salt cubic blocks, roughly 10 cm on a side), both uncaged and caged to exclude predators (crabs, fishes, whelks, and octopi), were deployed along a decreasing vent fluid-flux gra- dient. The exclusion of predators for 8 mo increased the abundance of small mobile gas- tropods and amphipod crustaceans but decreased the abundance of sessile invertebrates, including juvenile vestimentiferan worms, tubiculous polychaetes, and mussels. Effects of predation were strongest nearest to hydrothermal vents, where abiotic environmental con- ditions were most extreme but productivity and the overall abundances of benthic inver- tebrates and mobile predators were the greatest. Additional 5-mo experiments conducted at three different locations showed similar trends at all sites, indicating that these effects of predation on benthic community structure are repeatable. Stomach-content analyses of the most abundant predators found at vents indicated that the zoarcid fish ( Thermarces cerberus) primarily feeds on the vent snail Cyathermia naticoides, the limpet Lepetodrilus elevatus, and the amphipod crustacean Ventiella sulfuris, the very species that showed the greatest increase following predator exclusion. In contrast, brachyuran ( Bythograea ther- mydron) and galatheid (Munidopsis subsquamosa ) crab stomachs did not contain small mobile grazers, and crabs presented with arrays of the most common vent invertebrate species preferred mussels and vestimentiferans over limpets. Our results indicate that pre- dation by large mobile predators influences the structure of hydrothermal vent communities, directly by reducing the abundance of gastropod prey species, and indirectly by reducing

The use of biofouling for ageing lost nets: a case study

Abstract The use of biofouling for estimating the age of lost nets was investigated by means of net loss simulations and sequential retrieval, conducted in the Bay of Biscay (northern Spain) and off the Algarve (southern Portugal). Two trials were conducted in each location at different seasons. At both study sites, heavier colonisation was observed on the upper parts of the retrieved nets than in the bottom sections, irrespective of season. The biomass of fouling organisms was subject to geographical and seasonal variation, being considerably higher in the Algarve and throughout the spring/summer trials. Three bivalve species were selected as key species for net ageing: Anomia sp., Hiatella artica and Pteria hirundo . Bivalve abundance increased over time, until the 7th month of the trials in the Bay of Biscay and the 3rd month off the Algarve, and decreased thereafter. Analyses of the specimens’ size evolution over time indicated that the shell length of Anomia sp. could be used for ageing abandoned nets in the Bay of Biscay and that of P. hirundo off the Algarve. However, the use of these particular organisms for estimating soaking times of fishing gear is limited to a specific set of environmental conditions.

Feeding Ecology of the Sandbar Shark in South Carolina Estuaries Revealed through δ13C and δ15N Stable Isotope Analysis

Abstract Stable isotope ratios of carbon and nitrogen (δ13C and δ15N) from muscle samples were used to examine the feeding ecology of a heavily exploited shark species, the Sandbar Shark Carcharhinus plumbeus. Two hundred and sixty two Sandbar Sharks were sampled in five South Carolina estuaries. There were no significant differences in average δ13C or δ15N signatures between estuaries, between sampling years, or between male and female Sandbar Sharks, suggesting that these variables do not affect diet. A potential ontogenetic diet shift between young-of-year and juvenile Sandbar Sharks in South Carolina, similar to a shift previously described in Virginia and Hawaii populations, is suggested by significant differences in average δ13C and average δ15N signatures between these age-classes. Results confirm that Sandbar Sharks in South Carolina are generalist predators and that juvenile Sandbar Sharks have a wider diet breadth than young-of-year sharks, a pattern common in elasmobranchs. Sandbar Shark diet in South Carolina is similar to that found in previous stomach content analysis studies. This study also demonstrates that nonlethal sampling methods can be applied to sharks to obtain diet and trophic information, including the detection of ontogenetic shifts in diet.

Trophic Ecology of a Predatory Community in a Shallow-Water, High-Salinity Estuary Assessed by Stable Isotope Analysis

Abstract Estuaries serve as habitats and nurseries for many recreationally and commercially important fishes, often contributing recruits to adult populations that remain in close proximity to estuarine environments. Upper-level predatory fish species are among the most sought after by fisheries; thus, an understanding of the trophic dynamics of the community can assist ecological fisheries management of these highly productive ecosystems. Dietary niche overlap within the predatory fish community of Bulls Bay, a subtropical estuary in South Carolina, was assessed by using stable isotope analyses (δ13C and δ15N) to compare seven elasmobranch species and three teleost species. Cownose Rays Rhinoptera bonasus and Finetooth Sharks Carcharhinus isodon had no isotopic overlap with other species and therefore exhibited unique isotopic niche spaces that were indicative of potential resource partitioning. The teleosts and remaining elasmobranchs had varying degrees of overlap, implying shared resources; a high degree of dietary niche overlap was observed among Spotted Seatrout Cynoscion nebulosus, Sandbar Sharks Carcharhinus plumbeus, and Atlantic Sharpnose Sharks Rhizoprionodon terraenovae. Although most pairs of species showed some isotopic overlap, there were also interspecific differences in niche overlap, signifying that this predatory fish community has a widely varied prey base overall. Bulls Bay is an important nursery habitat with a balanced predator community, as illustrated by a combination of unique dietary niches and varying degrees of dietary niche overlap.

Influence of Central Pacific oceanographic conditions on the potential vertical habitat of four tropical tuna species

Abstract Climate change has resulted in the geographic and vertical expansion of oxygen minimum zones but their impact on the vertical distribution of commercially important species, such as tunas, is not well understood. Although La Niña events are characterized by increased upwelling along the equator, the increased primary productivity and bacterial proliferation drive the expansion of oxygen minimum zones. Vertical habitat of four tropical tuna species were characterized using direct observations of the oceanographic conditions of the Central Pacific Ocean during the 2008 La Niña event and existing primary literature on temperature and dissolved oxygen physiological tolerances for these tunas. Concentrations of potential prey were estimated using Acoustic Doppler Current Profiler raw backscatter and surface zooplankton tows. Based on the oceanographic conditions observed from February to June, low dissolved oxygen levels, more so than low temperatures, were inferred to restrict the predicted vertical habitat of four commercially important tuna species (bigeye, yellowfin, skipjack, and albacore). During peak La Niña conditions, temperature and dissolved oxygen tolerance limits of all four tuna species were reached at approximately 200m. Zooplankton and myctophid fish densities peaked in the upper 200m between 0° N and 5° N, which corresponded to a region with a shallow thermocline (150 m). Our findings suggest the possibility that competition and susceptibility to surface fishing gears may be increased for tropical tunas during a strong La Niña event due to vertical habitat restrictions.