Evgeny V. Romanov

High connectivity of the crocodile shark between the Atlantic and Southwest Indian Oceans: highlights for conservation.

Among the various shark species that are captured as bycatch in commercial fishing operations, the group of pelagic sharks is still one of the least studied and known. Within those, the crocodile shark, Pseudocarcharias kamoharai, a small-sized lamnid shark, is occasionally caught by longline vessels in certain regions of the tropical oceans worldwide. However, the population dynamics of this species, as well as the impact of fishing mortality on its stocks, are still unknown, with the crocodile shark currently one of the least studied of all pelagic sharks. Given this, the present study aimed to assess the population structure of P. kamoharai in several regions of the Atlantic and Indian Oceans using genetic molecular markers. The nucleotide composition of the mitochondrial DNA control region of 255 individuals was analyzed, and 31 haplotypes were found, with an estimated diversity Hd = 0.627, and a nucleotide diversity π = 0.00167. An analysis of molecular variance (AMOVA) revealed a fixation index Φ ST = -0.01118, representing an absence of population structure among the sampled regions of the Atlantic Ocean, and between the Atlantic and Indian Oceans. These results show a high degree of gene flow between the studied areas, with a single genetic stock and reduced population variability. In panmictic populations, conservation efforts can be concentrated in more restricted areas, being these representative of the total biodiversity of the species. When necessary, this strategy could be applied to the genetic maintenance of P. kamoharai.

Variability in conspecific predation among longnose lancetfish Alepisaurus ferox in the western Indian Ocean

Conspecific predation among longnose lancetfish Alepisaurus ferox was investigated in four spatio-temporal strate of the western Indian Ocean. The cannibalism level varied from 0 to 45.5% by frequency of occurrence and was negatively related with abundance of non-evasive prey (such as crustaceans Charybdis smithii and Natosquilla investigatoris) and foraging success. Predation by lancetfish is often described as a non-selective process, constrained by local prey availability and by its feeding speed during an attack of prey. Our results show that lancetfish may adapt its opportunistic foraging behavior, feeding on non-conspecific abundant prey such as crustaceans when available, and switching to a high level of conspecific predation in poor waters.

Stranding and mortality of pelagic crustaceans in the western Indian Ocean

Recent observations of unusual mass stranding and mortality of two Indian Ocean crustacean species, the swimming crab Charybdis smithii and the mantis shrimp Natosquilla investigatoris, are documented and analysed. Strandings of C. smithii were observed for the first time in the equatorial Indian Ocean, the main area of its pelagic distribution. Strandings of mantis shrimps are reported from throughout the western Indian Ocean; occurrences of mass stranding in the Maldives Archipelago mark an extension of the known range of N. investigatoris into the central Indian Ocean. Mortality of crabs probably represents a catastrophic event. In contrast, mantis shrimp strandings, which were always associated with a sudden increase of its biomass (blooms), are apparently post-reproduction mortalities indicating potential semelparity for this species.

Comment on "Clarification regarding the distribution of bigeye tuna Thunnus obesus in the Atlantic Ocean, including British waters":

Powell et al. (2009) recently reported the occurrence of a specimen of Thunnus obesus (Lowe) found stranded on 24 August 2006 near Burry Port, Wales, U.K. (51◦ 40′ N; 4◦ 15′ W). The authors describe the biological characteristics of the fish such as morphometric measurements and stomach contents, the diagnostic anatomical features for identifying T. obesus and discuss the past occurrences of tropical and temperate tunas in the Celtic Sea and adjacent areas. Powell et al. (2009) finally suggest that the occurrence of tropical and sub-tropical fish species around the British Isles could be related to the significant increase in sea temperature in the recent decades. While it is accepted that changes in the geographic distribution of fish populations due to climate change are possible and an important area of research (Stebbing et al., 2002; Perry et al., 2005; Poulard & Blanchard, 2005), the paper of Powell et al. (2009) includes several inconsistencies that severely limit the reliability of their findings and are mainly due to a lack of knowledge of the ecology and fisheries of T. obesus. A large amount of biological and ecological information and fisheries data have been collected since the early 1950s by national research institutes and fisheries administrations on large pelagic species of the Atlantic Ocean and adjacent seas. The International Commission for the Conservation of Atlantic Tunas (ICCAT; www.iccat.int) is the regional marine fisheries organization in charge of the study and management of these species. The ICCAT contracting and cooperating noncontracting parties annually provide for each fleet and gear the catch composition (in mass and number of fishes) obtained by a given amount of fishing effort in a given spatial and temporal unit. Data and associated information are freely available online through the ICCAT website and annually published in the Collective Volume of Scientific Papers ICCAT series, biennial reports and statistical bulletins.

Toothed whale and shark depredation indicators: A case study from the Reunion Island and Seychelles pelagic longline fisheries

Depredation in marine ecosystems is defined as the damage or removal of fish or bait from fishing gear by predators. Depredation raises concerns about the conservation of species involved, fisheries yield and profitability, and reference points based on stock assessment of depredated species. Therefore, the development of accurate indicators to assess the impact of depredation is needed. Both the Reunion Island and the Seychelles archipelago pelagic longline fisheries targeting swordfish (Xiphias gladius) and tuna (Thunnus spp.) are affected by depredation from toothed whales and pelagic sharks. In this study, we used fishery data collected between 2004 and 2015 to propose depredation indicators and to assess depredation levels in both fisheries. For both fisheries, the interaction rate (depredation occurrence) was significantly higher for shark compared to toothed whale depredation. However, when depredation occurred, toothed whale depredation impact was significantly higher than shark depredation impact, with higher depredation per unit effort (number of fish depredated per 1000 hooks) and damage rate (proportion of fish depredated per depredated set). The gross depredation rate in the Seychelles was 18.3%. A slight increase of the gross depredation rate was observed for the Reunion Island longline fleet from 2011 (4.1% in 2007–2010 and 4.4% in 2011–2015). Economic losses due to depredation were estimated by using these indicators and published official statistics. A loss of 0.09 EUR/hook due to depredation was estimated for the Reunion Island longline fleet, and 0.86 EUR/hook for the Seychelles. These results suggest a southward decreasing toothed whale and shark depredation gradient in the southwest Indian Ocean. Seychelles depredation levels are among the highest observed in the world revealing this area as a “hotspot” of interaction between pelagic longline fisheries and toothed whales. This study also highlights the need for a set of depredation indicators to allow for a global comparison of depredation rates among various fishing grounds worldwide.

Is skipjack tuna the fastest-growing teleost fish on earth?

Using a genetic marker mitochondrial DNA control region, Nikolic et al. (Environ Biol Fish 99:171-178, 2016) recently reported the identification of two juveniles of tuna as skipjack (Katsuwonus pelamis) recovered from the mouth of dolphinfish in the vicinity of Reunion Island, western Indian Ocean. Based on the age estimates of the two specimens from opercular bones at about 16–20xa0days and the increased occurrence of skipjack in the catch of Japanese longliners during October–January, the authors indicate that their results suggest a specific seasonal spawning area of skipjack around Reunion Island. We challenge the validity of age estimates and the interpretation of the results published by Nikolic et al. (Environ Biol Fish 99:171-178, 2016). Current knowledge of skipjack tuna biology indicates that the paper of Nikolic et al. (Environ Biol Fish 99:171-178, 2016) contains misinterpretation of field observations that led the authors to misunderstandings on skipjack tuna ecology.