Community-level impacts of trawl selectivity in the Eastern Mediterranean Sea assessed using an ecosystem modelling approach

Switching from the traditional 40- or 44-mm diamond mesh codends to 50-mm diamond mesh (D50) or 40-mm square mesh (S40) codends is known to improve the size selection for commercial species and reduce discarding in the Mediterranean demersal trawl fisheries. This change has been recommended in the General Fisheries Commission for the Mediterranean and European Commission regulations. However, ecosystem-based assessment of improved selectivity still remains a challenge, and that is the aim of this study. For this purpose, an Ecopath model was developed and used to initialize dynamic simulations in Ecosim. The simulations changed gear-specific fishing mortality rates to represent the shifting from traditional codend to alternative codends. Our results suggest that the use of D50 or S40 codends would have a positive impact on the ecosystem as a whole and on the stock status of several commercial and non-commercial groups. Mixed trophic impact analysis indicated that, for species, like red mullet, the S40 codend was significantly better than the D50 codend. This information can be useful to policymakers, particularly for the areas where red mullet is the main target species and could inform trawl mesh options in the eastern Mediterranean.

A first look at the metabolic rate of Greenland sharks (Somniosus microcephalus) in the Canadian Arctic

Metabolic rate is intricately linked to the ecology of organisms and can provide a framework to study the behaviour, life history, population dynamics, and trophic impact of a species. Acquiring measures of metabolic rate, however, has proven difficult for large water-breathing animals such as sharks, greatly limiting our understanding of the energetic lives of these highly threatened and ecologically important fish. Here, we provide the first estimates of resting and active routine metabolic rate for the longest lived vertebrate, the Greenland shark (Somniosus microcephalus). Estimates were acquired through field respirometry conducted on relatively large-bodied sharks (33–126 kg), including the largest individual shark studied via respirometry. We show that despite recording very low whole-animal resting metabolic rates for this species, estimates are within the confidence intervals predicted by derived interspecies allometric and temperature scaling relationships, suggesting this species may not be unique among sharks in this respect. Additionally, our results do not support the theory of metabolic cold adaptation which assumes that polar species maintain elevated metabolic rates to cope with the challenges of life at extreme cold temperatures.

Spatially Varying Trophic Effects of Reservoir-Derived Plankton on Stream Macroinvertebrates Among Heterogeneous Habitats Within Reaches

SummaryDam reservoirs often supply high amounts of plankton to downstream reaches, leading to a critical shift of trophic origins of stream ecosystems from natural sources (e.g. attached algae and terrestrial inputs) to reservoir-oriented plankton. Although this is a widely observed phenomenon, previous studies focused only on lotic habitats (e.g. riffles) rather than lentic habitats such as backwaters and isolated ponds (IP).Using a stable isotope three-source mixing model, we evaluated trophic contributions of reservoir-derived plankton, epilithon and terrestrial leaves to stream macroinvertebrates at four dam outlet reaches and two reference reaches in the Natori River catchment, Japan. We compared four different habitat types co-occurring within the reaches: lotic habitat (riffle and pool), bar-head (BH) lentic habitat, bar-tail (BT) lentic habitat (backwater) and isolated pond (IP) on sandy bars.The trophic contributions of reservoir-derived plankton were significantly lower in lentic habitats (BH, 15.4%; BT, 10.4%; IP, 9.1%) than in lotic habitats (mean, 27.7%). This was especially notable for filter feeders that feed on suspended fine particulate organic matter (SFPOM). The three-source model analysis indicated a lower biomass proportion of dam plankton in lentic SFPOM (mean, 21.2%) than in lotic SFPOM (mean, 35.6%). This difference in SFPOM composition was reflected in the lower trophic contribution of dam plankton to lentic filter feeders.The abundance ratio of filter feeders in the community was decreased in lentic habitats, while the abundance ratios of collector-gatherers, scrapers and shredders were increased. Macroinvertebrates in lentic habitats fed on sources less mixed with reservoir-derived plankton (e.g. benthic coarse particulate organic matter [BCPOM], benthic fine particulate organic matter [BFPOM] and epilithon); therefore, the trophic impact of reservoirs was indistinctive at the community level, indicating that lentic habitats can function as trophic refugia to mitigate the trophic impact of reservoirs.Because lentic habitats were decreased in area (accounting for 5.7% of average total area) in the downstream reaches of dams due to riverbed degradation, lentic habitats must be created in order to restore the trophic impact of reservoirs in river ecosystems.

Dynamics of a Heterotrophic Dinoflagellate, Protoperidinium divergens, in the South-Eastern Coastal Waters of the Bay of Bengal

This research is the first attempt to present temporal distribution of heterotrophic dinoflagellate Protoperidinium divergens and their relationship with diatom and environmental factors in the Maheshkhali channel, south-east coast of the Bay of Bengal, Bangladesh. The initiation of higher abundances of P. divergens were observed in October and reached its peak in November when diatoms were the most abundant phytoplankton. Protoperidinium divergens preferred comparatively low temperature from 20-22°C and high concentration of phosphate-phosphorus from 31-37 µM. It had insignificant relationship with nitrate-nitrogen. Lower abundances of P. divergens were observed during pre-monsoon and monsoon due to lower abundances of diatom and higher water temperature (>23°C). Protoperidinium divergens, therefore, were mainly controlled by the availability of diatom abundance at comparatively lower temperature and higher concentration of phosphate-phosphorus. This research is important to estimate the potential trophic impact of P. divergens in the Maheshkhali channel.

What’s for dinner? Diet and potential trophic impact of an invasive anuran Hoplobatrachus tigerinus on the Andaman archipelago

Amphibian invasions have considerable detrimental impacts on recipient ecosystems. However, reliable risk analysis of invasive amphibians still requires research on more non-native amphibian species. An invasive population of the Indian bullfrog, Hoplobatrachus tigerinus, is currently spreading on the Andaman archipelago and may have significant trophic impacts on native anurans through competition and predation. We carried out diet analyses of the invasive H. tigerinus and native anurans, across four habitat types and two seasons; we hypothesized that (i) small vertebrates constitute a majority of the H. tigerinus diet, particularly by volume and (ii) the diet of H. tigerinus significantly overlaps with the diet of native anurans, thereby, leading to potential competition. We assessed the diet of the invasive H. tigerinus (n = 358), and individuals of the genera Limnonectes (n = 375) and Fejervarya (n = 65) and found a significant dietary overlap of H. tigerinus with only Limnonectes. Small vertebrates, including several endemic species, constituted the majority of H. tigerinus, diet by volume, suggesting potential impact by predation. Prey consumption and electivity of the three anurans indicated a positive relationship between predator-prey body sizes. Individuals of H. tigerinus and Fejervarya chose evasive prey, suggesting that these two taxa are mostly ambush predators; individuals of Limnonectes chose a mixture of sedentary and evasive prey indicating that the species employs a combination of ‘active search’ and ‘sit and wait’ foraging strategies. All three species of anurans mostly consumed terrestrial prey. This intensive study on a genus of newly invasive amphibian contributes to knowledge of the impact of amphibian invasions, and elucidates the feeding ecology of H. tigerinus, and species of the genera Limnonectes and Fejervarya. We also stress the necessity to evaluate prey availability and volume in future studies for meaningful insights into diet of amphibians.