E V Romanov - Bycatch in the soviet purse seine tuna fisheries on fad-associated schools in north equatorial area of the western indian ocean - страница 1

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ТРУДЫ ЮЖНОГО НАУЧНО-ИССЛЕДОВАТЕЛЬСКОГО ИНСТИТУТА МОРСКОГО РЫБНОГО ХОЗЯЙСТВА И ОКЕАНОГРАФИИ, 2000, Т. 45

PROCEEDINGS OF THE SOUTHERN SCIENTIFIС RESEARCH INSTITUTE OF MARINE FISHERIES & OCEANOGRAPHY, 2000, VOL.45

EVGENY V. ROMANOV

BYCATCH IN THE SOVIET PURSE SEINE TUNA FISHERIES ON FAD-ASSOCIATED SCHOOLS IN NORTH EQUATORIAL AREA OF THE WESTERN INDIAN OCEAN[1]

В международном кошельковом промысле тунцов в западной части Индийского океана целевыми видами являются желтоперый и полосатый тунцы. Более 40 видов рыб и других морских животных также встреча­ются в уловах кошельковых неводов вместе с тунцами и являются приловом. В работе оцениваются величины прилова советского/россий­ского/либерийского кошелькового флота в Северном Экваториальном подрайоне Индийского океана (0-10° с.ш., 45-70° в.д.) в течение сезона промысла у искусственных концентрирующих устройств (FAD) (август-ноябрь) и выбросы, связанные с этим промыслом. Данные были собраны научными наблюдателями на борту сейнеров, работавших под флагом СССР/России/Либерии в западной части Индийского океана (WIO) в течение 1986-1992 гг. Проанализировано 108 заметов на ассоциированные с FAD косяки. Средняя величина прилова составляет 1.923 т за замет (нетунцовыйприлов 0.915т), или 96.8 т (46.1 т) на 1000 т целевых видов. Основные виды в прилове - большеглазый тунец (0.995 т за замет), пелагические океанские акулы (0.246 т), элагат (0.215т), курки (0.199 т) и корифены (0.169 т). Зарегистрирован единичный случай прилова морс­кой черепахи. Величина выбросов оценена на уровне 0.891 т за замет или 44.9 т на 1000 т целевых видов. Потенциальные выбросы (которые включают в себя мелкого полосатого, желтоперого и большеглазого тунцов, а также макрелетунца и ауксиду всех размеров) оценены на уровне 0.162 т за замет или 8.3 т на 1000 т целевых видов.

The bycatches taken by the Soviet/Russian/Liberian tuna purse seiners from the North Equatorial Area (0-10°N, 45-70°E) of the Indian Ocean during FAD (fish aggregation devices) fishing season (August-November) and discards resulted from this fishery were estimated. Data were collected by scientific observers aboard Soviet/Russian/Liberian-flag purse seiners in the western Indian Ocean (WIO) during 1986-1992. A total of 108 sets on FAD-associated schools were analysed. More than 40 fish species and other marine animals were recorded, of which only two species, yellowfin and skipjack tunas, are target species. Average levels of bycatch were 1.923 metric tons (t) per set (non-tuna bycatch 0.915 t), or 96.8 t (46.1 t) per 1,000 t of target

species. Principal species in the bycatch were bigeye (0.995 t per set), pelagic oceanic sharks (0.246 t), rainbow runner (0.215 t), triggerfishes (0.199 t), and dolphinfishes (0.169 t). One turtle was recorded in the bycatch. Estimated discards are equal to 0.891 t per set or 44.9 t per 1000 t of target species. Potential discards (which included small skipjack, yellowfin, bigeye, and all frigate and bullet tunas) were estimated on the level 0.162 t per set or 8.3 t per 1000 t of target species.

INTRODUCTION

Two tunas, yellowfin Thunnus albacares (Bonnaterre, 1788) and skipjack Katsuwonus pelamis (Linnaeus, 1758), are the target species of western Indian Ocean purse-seine tuna fisheries. More than 40 species of fish and other marine animals regularly or occasionally occur in the catches together with this two tunas forming bycatch.

In this paper bycatch is defined as the fraction of the catch that consists of non-target species (including other species of tuna), which are encircled by the fishing gear and are unable to escape by themselves. Bycatches may be retained on board or discarded. Bycatches of associated and non-associated species during purse-seine fishing for tropical tunas may be rather high, and generally depend on fishing tactics (i. e. number of set on different types of surface tuna schools: free-swimming, associated with marine mammals, log-(or FAD-) associated, etc.).

The FAD fishing technique (sets on natural logs, anthropogenic flotsam, man-made FAD) were introduced in different purse seine tuna fisheries for different reasons: to improve catch rates, minimize fishery expenses, to comply with «dolphin-safe» policy etc. Such fishing tactics may produce relatively high bycatch rates [1-5].

In the Indian Ocean the purse seine tuna FAD fishing technique was used from the early years of the fishery and expanded extensively in recent years (starting from 1995). Purse seiners make FAD sets in the WIO throughout the year but the principal season of FAD fisheries occurs in the summer-autumn season in the North Equatorial area (0-10°N, 45-70°E).

No comprehensive bycatch estimates were published for the fishery up to the present[2]. Some information on bycatch levels and species composition was presented by Stretta et al. [10] and Santana et al. [8].

This paper is an attempt to estimate bycatch and discards by Soviet/ Russian/Liberian tuna purse seiners of in the WIO, during the principal season of FAD fisheries in the North Equatorial area, based on information collected by scientific observers.

MATERIALS AND METHODS

Sampling methodology and the methods of analysis were described in Romanov [7]. Extract from «Materials and methods» from Romanov [7] is attached in Appendix I.

FAD in this paper means any type of floating object used for tuna fishing (natural logs, palm branches, anthropogenic flotsam (table desk, freezers,

wire rope bobbins, longline floats, etc.), specially constructed fish aggregation devices, etc.).

The majority of the sampled data does not consist of any information on discards. Recorded discards discussed in the «Discards» chapter. In order to obtain additional information on discards I interwiewed several observers and one member of the tuna vessel crew (fish processing officer) for their estimates of discards. Results are presented in the «Discards» section below.

Size frequencies of principal tuna species were raised to total sampled catch. In the raising procedure for estimates of total weight of tuna measured (if sample not weighed aboard) the following L-W relationships were used (calculated by the author basing on YugNIRO database):

Yellowfin W=3.111907*10-5*L2.859513 (n=16240)

Skipjack W=5.173059*10-6*L3.339043 (n=6990) Bigeye W=1.882289*10-5*L2.980910 (n=1374).

For albacore, frigate and bullet tunas no representative size frequencies sampling coverage were recorded.

RESULTS AND DISCUSSION

Total catch and sampled catch distribution

Detailed total catch and catch composition data for Soviet/Russian/ Liberian purse seine tuna vessels by small-scale strata are available for 1985­1994. These figures are based on daily radioreports on fishing activity of the vessels. The catch reporting rate according to my estimates varied in the range 96-99% during 1985-1991, decreasing to 71% in 1992-1994 [7].

Total catches of the Soviet fleet in 1985-1994, sampled catch and sampled FAD catch (two latter is grouped for the same 10-years time span) by one-degree stratum are presented at Fig. 1. The same pictures of catch distribution during August-November given at Fig. 2.

For the purpose of analysis of sampling representativeness, cumulative (August-November) catches in 1985-1994 in the North Equatorial area 0-10°N, 45-75°E were grouped by five-degree squares. Catch was recorded in 9 of the 10 five-degree squares (Fig. 3). Every 5-degree substratum with catch was numbered from 1 to 9 respectively (Fig. 3). Total Soviet catch varied considerable within this area. Highest catch was recorded in strata 6, 7, and 8 representing in total 91% of total catch. Fig. 3 shows total catch by 5-degree square, the share of sampled catch in 1986-1992 to cumulative catch by stratum and the percentage of FAD sets in the sampled catch. Areas with highest fishing activity: 6, 7, 8 were sampled at the rate from 6 to 24% (in weight). Non-sampled areas: 1, 2, 4, 5 correspond to areas with minor fishing activity (about 5% of total catch). Proceeding from this I believe that sampled data adequately represent catch and fishery activity in the area/season.

A total of 108 purse-seine FAD sets were sampled and 101 positive sets were analysed in the North Equatorial Area, which correspond to 87% of total number of sampled sets in the area/strata. The total catch in the FAD sets that were sampled amounted to 2200 t.

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E V Romanov - Bycatch in the soviet purse seine tuna fisheries on fad-associated schools in north equatorial area of the western indian ocean