Mediterranean Bottom Trawl Research Articles

Performance of a northwestern Mediterranean bottom trawl fleet: How the integration of landings and VMS data can contribute to the implementation of ecosystem-based fisheries management

The European Union has established a framework to achieve or maintain good environmental status in the marine environment by 2020. The Marine Strategy Framework Directive requires the application of the ecosystem approach to the management of human activities, covering all sectors having an impact on the marine environment. However, fisheries in the Mediterranean are far from a systematic implementation of an ecosystem-based fisheries management (EBFM). Aiming to address this issue, this study explores the potential of the relationship between daily yield by vessel (landings and income by species) and vessel position (known via vessel monitoring system) as a tool for fleet management. This approach is possible due to the current dynamics of Mediterranean fleets, with vessels returning daily to the harbour where landings are registered as weight and income by vessel. Moreover, vessels of >15m total length have been compulsory monitored by VMS since 2005. A bottom trawl fleet that operates in the northwestern Mediterranean was chosen to develop this approach. Different groups of trawlers were identified, which could be linked to the strategies displayed by the fishermen that were mainly driven by the target species dynamics. Accurate knowledge of the fishing targets driving the fleet dynamics and of the fishing strategies at the vessel level (i.e. fishing ground habitat where the fishing pressure is exerted and corresponding landings) are shown to be a feasible tool for fleet management.

A Histological Study of Ovarian Development in the Giant Red ShrimpAristaeomorpha foliacea(Crustacea: Decapoda: Aristeidae) from the Southern Tyrrhenian Sea (Western Mediterranean)

The reproductive features of the giant red shrimp, Aristaeomorpha foliacea, were investigated in the southern Tyrrhenian sea by experimental trawl sampling. The annual length-frequency distribution showed a multimodal trend in females, ranging between 16 and 67 mm carapace length (CL), and a unimodal trend in males (18–45 mm CL). Mature males occurred in different proportions all year round, while females displayed seasonal maturity (June—September), with a peak in July. Six oocyte developmental stages were identified, the most advanced of which (Pv, postvitellogenic) had never been described before in this species. Ovary development followed a group-synchronous pattern, with the yolked oocyte stock clearly separated from the reservoir of unyolked oocytes, suggesting that A. foliacea is a total spawner, with determinate fecundity. Based upon histological findings, a revision of macroscopic maturity staging employed in Mediterranean bottom trawl surveys (MEDITS) is proposed.

Reproductive features of the deep-water rose shrimp, Parapenaeus longirostris (Crustacea: Penaeidae), in the Strait of Sicily

The deep‑water rose shrimp, Parapenaeus longirostris (Lucas, 1846), is one of the most valuable and heavily exploited demersal species of the Mediterranean bottom trawl fisheries. The basic life traits of this shrimp, in particular its reproductive aspects, are regularly monitored during experimental trawl surveys carried out in the Mediterranean Sea. Gonadic condition and maturity status for estimating the size at onset of sexual maturity are commonly assessed in females, using macroscopic color scales, histologically validated only in a few geographical areas. In this study, histological analyses were performed on rose shrimps collected from a trawl survey carried out in the Strait of Sicily, in order to support the empirical 4‑stage macroscopic scale locally employed. Ovaries from females of different sizes, ranging between 16 mm and 40 mm carapace length, were collected and used for microscopic examination of their structure, for oocytes counts and for oocyte diameter frequency distributions; oocytes diameter was measured by imaging analysis. The collected data were also used to estimate other basic vital parameters of the rose shrimp population. The histological observations show a broad correspondence between ovarian development and macroscopic features; therefore, the classifications derived by the empirical scale remain suitable for estimating the maturity parameters.

Efficiency of flexible sorting grids to improve size selectivity of the bottom trawl in the Balearic Islands (western Mediterranean), with comparison to a change in mesh cod-end geometry

In recent years several studies have been developed to improve the selectivity of the Mediterranean bottom trawl fisheries, which exert high fishing pressure on young individuals. The focus has been mainly on increasing the mesh size in the cod-end or changing its mesh geometry; few studies have assessed the efficiency of the sorting grid systems. Analysis of 21 trawl hauls carried out at depths of 117–697 m off the Balearic Islands (western Mediterranean) in October–November 2005, studied size selectivity of the trawl using flexible sorting grids with 15 (SG15) and 20 (SG20) mm bar spacing. A divided polyamide bottom trawl designed for commercial purposes demonstrated high efficiency for the simultaneous use of both sorting grid sizes at different depth intervals (DS: deep shelf; US: upper slope; MS: middle slope). The results were also compared with those from the authors’ previous studies in the area where 40 mm diamond (DM) and square (SM) mesh cod-ends were used. A saturation effect was detected on the DS for both sorting grids, due to the large amount of biomass captured at this depth interval. Size selectivity of 11 species was modelled, which showed an increase in length at first capture (L50) from SG15 to SG20. Values of L50 estimated for the main target species were: 10.9 cm total length (TL) with SG15, and 18.9 cm TL with SG20 for Merluccius merluccius; 21.1 mm carapace length (CL) with SG15, and 23.8 mm CL with SG20 for Nephrops norvegicus; and 15.9 mm CL with SG15, and 19.6 mm CL with SG20 for Aristeus antennatus. Comparison of size selectivity parameters between the sorting grids and cod-ends of different geometry showed clear differences. For most species, the highest value of L50 was obtained with SM (e.g. 26.6 mm CL for N. norvegicus and 22.1 mm CL for A. antennatus). Exceptions were the round-fish M. merluccius, the flatfish Lepidorhombus boscii, and the crustacean Parapenaeus longirostris, with values of SG20 (18.9 cm TL, 12.1 cm TL, and 25.7 mm CL, respectively) higher than with SM (15.2 cm TL, 10.2 cm TL, and 20.2 mm CL, respectively). Sorting grid selection ranges were broader than those estimated for DM and SM. Experience shows that the exchange of the DM for the SM and/or the introduction of SG20 can improve selectivity in the Mediterranean trawl, SM being more efficient than SG20 in the Balearic Islands.

A “Traffic” Light approach for the assessment of the Broadtail shortfin squid Illex coindetii Verany, 1839 in the Southern Adriatic Sea (Central Mediterranean)

A multi-indicator system and the Traffic Light methodology (TL) to provide the first description of the Illex coindetii (Verany 1839) stock in the GFCM geographical sub-area (GSA) 18 (southern Adriatic Sea; Central Mediterranean) were applied using independent fishery data (“Mediterranean Bottom Trawl Survey”, “Medits”, programme, years 1996–2004). The trajectories of nine population indicators of Frequency of Occurrence (F), Biomass (BI), Density (DI), Recruit Strength (RS), Spawner Strength (SS), Mean Body Weight (MBW), Mantle Length (ML), Mantle Length Excluding Recruits fraction (MLER), Mean mantle Length-Females Length at maturity ratio (MLLm), were analysed using statistical estimators as arithmetic and geometric mean, median, and 75th percentile. The TL methodology was applied to analyse the resulting multiple-indicator panel. Results describe a significant trend of the I. coindetii stock (in number) in the GSA 18 related to a shift within its structure due to a greater proportion of small specimens in the population. Factors that may be contributing to the observed changes are sea temperature, the reduction of possible predators, the recovery of some prey species mostly due to the fishing pressure occurring in the area, and the fast growth and short life span of I. coindetii. In a dynamic system like the Adriatic Sea, the capability to respond quickly to the “environmental” or “ecosystem” changes seem to give I. coindetii an advantage in competition with the long lived finfish species. Accordingly, there is potential to use squid population dynamics as recorders to track environmental and ecological changes, as well as overexploitation. The adopted methodology drew attention to changes occurring within the stock of I. coindetii, and could provide an opportunity for managers to consider action at an early stage. The multi-indicator system and the TL methodology are illustrated as means to follow dynamic changes within squid stock, and to provide useful information for the assessment of the resource.

The influence of twine thickness on the size selectivity of polyamide codends in a Mediterranean bottom trawl

We investigated the effect of twine thickness on the codend selectivity of Mediterranean bottom trawls. Two codends having meshes with the same opening (ca. 44 mm) but different twine thicknesses (Light PA: R3644tex, Ø 2.38 mm and Heavy PA: R5312tex, Ø 2.89 mm) were fished daily and alternately on the same trawl. Selectivity was measured using the covered codend technique, the cover being supported by circular hoops. The results were analysed taking into account the between-haul variation in selectivity. They demonstrated a substantial reduction in selectivity with increased twine size. For all the species considered, the lengths at 50% retention (L50) indicate that an increase in twine thickness from 2.38 mm (Rtex 3644) to 2.89 mm (Rtex 5312) reduces selectivity by 20–31%. Therefore, the twine thickness of the codend netting played an important role in selectivity. It is thus essential to include this factor in EU legislation aimed at improving codend selectivity in Mediterranean bottom trawl fisheries, where the recent trend towards using thicker twines threatens to cancel the benefit of the effect of mesh size increases or that of the square-mesh codend, allowing the present high rate of discarding small demersal fish to continue.

Trends in the abundance of demersal species in the southern Aegean Sea

The present work identifies trends in the abundance of the main demersal species of the South Aegean Sea utilising catch and effort data derived from an international Mediterranean bottom trawl survey (MEDITS). The experimental surveys were accomplished annually from 1994 to 2000 on 61 pre-defined sampling stations. Selection of stations was based on a depth-stratified random sampling scheme that included five depth strata: 10-50, 50-100, 100-200, 200-500 and 500-800 m. The results suggest the existence of a general increasing trend in the biomass of the demersal species, particularly at the 10-50 m and 500-800 m depth strata. Findings are discussed in relation to the fisheries exploitation pattern and productivity of the area.