Trammel Net Fishery Research Articles

Elasmobranch captures in shrimps trammel net fishery off the Gulf of Gabès (Southern Tunisia, Mediterranean Sea)

Summary Small-scale fisheries are generally promoted as a sustainable alternative to large-scale industrial fisheries. However, there is recent growing evidence that small-scale fisheries may be the largest threat to marine species of conservation concern. The objective of this study was to evaluate the potential impact of the trammel net fishery on elasmobranchs in the Gulf of Gabes, Southern Tunisia. Data are based on 191 shrimp trammel net set (40 mm stretched mesh size) surveys conducted aboard commercial fishing vessels from May to July 2009. Five species of the small coastal elasmobranchs (Mustelus mustelus (Linnaeus, 1758), Mustelus punctulatus Risso 1827, Dasyatis pastinaca (Linnaeus, 1758), Dasyatis marmorata (Steindachner, 1892) and Torpedo torpedo (Linnaeus, 1758)) and two species from the large coastal shark (Carcharhinus plumbeus (Nardo, 1827) and Carcharhinus brevipinna (Muller & Henle, 1839)) were recognized as by-catch in this fishery. Elasmobranch by-catch was dominated by sharks (90.3%), smoothhound sharks Mustelus sp. being by far the most important (88.9%) and reflecting their abundance in the area; 58% of the sets caught at least one specimen, with 4.8 ± 1.3 caught per set. Captures were composed essentially of neonate and juvenile sharks, while the batoids were dominated by mature individuals. This study shows that shrimp trammel nets represent a considerable source of mortality for early life stages of elasmobranch species in the Gulf of Gabes. Additionally, there was a high density of neonates and small juvenile M. mustelus in the Sfax zone, suggesting that these nearshore waters are a nursery grounds for smoothhound sharks. Further research should focus on the incidents of by-catch and evaluate the potential solutions to allow trammel net fisheries to coexist alongside the elasmobranch species.

Sensitivity assessment as a tool for spatial and temporal gear-based fisheries management

Assessment of ecosystem health is required in the ecosystem approach to fisheries management (EAFM). Mitigation measures that address impacts exceeding the acceptable level also require assessment. The various fishing métiers have different ecosystem impacts and this makes it difficult to assess them jointly. Sensitivity of the ecosystem to individual fishing métiers has been assessed previously, but to our knowledge, concurrent comparisons of different métiers on the same ecosystem have not been done. In this study, we combined the main characteristics of established approaches to sensitivity assessment into a single roadmap, called Sensitivity Assessment of Gear Effects (SAGE). SAGE is a widely applicable, three-step process to assess the ecological concerns of EAFM. The methodology used in the SAGE roadmap is built on a scoring system, which then results in a sensitivity index of the ecosystem components to the fishing métiers. The scoring system is based on a combination of expert judgement and data, both qualitative and quantitative. It allows for cross-evaluation of fishing métiers and ecosystem components. Sensitivity maps are created using the spatial and temporal distribution patterns of the ecosystem components in the index. The uncertainty of the sensitivity scores and maps are estimated through a pedigree index. The index is based on proxy representation, empirical basis, methodological rigour, theoretical understanding and degree of validation, all of which measure the strength of the research results. The proposed methodology is illustrated using a case study that compares the ecosystem effects of beam trawl and trammel net fisheries. The selected examples did not result in unexpected outcomes, but were rather chosen to evaluate the applicability of our methodology. They illustrate how a semi-quantitative framework, which includes the uncertainties associated with scientific assessments, can deliver holistic advice to fisheries managers in a fully transparent manner.

Using guarding net to reduce regularly discarded invertebrates in trammel net fisheries operating on seagrass meadows (Posidonia oceanica) in Izmir Bay (Eastern Aegean Sea)

Prohibition of both beach and boat seines and trawl fishery along the İzmir Bay coasts in the Aegean Sea signifies intensive usage of gillnets and trammel nets, for catching red mullet (Mullus spp.) species in particular. Trials were realized between March 2009 and February 2010 with trammel nets in the areas on the boundaries of the sea grass (Posidonia oceanica) meadows in the Bay. Guarding net (selvedge) was attached to the lead line of experimental nets (Exp1-Exp2) - 36 and 40 mm inner panel. Differences for discard amounts between control group nets (C1-C2) (having the same inner panel as the experimental nets), used by commercial fishermen, and experimental nets are 54.7% for C1-Exp1 and 62.8% for C2-Exp2 (p<0.05). Use of nets with selvedge not only reduced regularly discarded invertebrates (Hexaplex trunculus, Bolinus brandaris, Maja spp.) in the region, but also avoided net damage caused by these species.

Climate-induced changes in fish landings of different fleet components of Portuguese fisheries

For each Portuguese fleet component, landings of biogeographic groups of fish species were compared for the period of 1993–2009. Wide-distribution species were the most abundant in landings, but have shown a decreasing trend. Temperate species had higher landings in trawl fisheries, whereas subtropical species were most abundant and exhibiting an increasing trend in landings of multi-gear fisheries. A latitudinal gradient was observed, with landings of temperate species being more important in the North-western coast than in the South-western and South coasts. Although trawl fisheries were relatively more important in the North-western coast, there has been a recent increase in the relative importance of multi-gear fisheries and of subtropical species in this area. The increasing relative importance of subtropical species in Portuguese fisheries along with the fact that landings of subtropical species were higher in multi-gear fisheries could indicate an easier adaptation of Portuguese multi-gear fisheries to the effects of climate change. However, as multi-gear fisheries include a wide range of gears, techniques and target-species, they may not all respond in the same manner to changes in fish species’ distribution. Among multi-gear fisheries, trammel nets catch a wider variety of species and a wider size range than gill nets or longlines; thus, trammel net fishers can adapt to changes in abundance of the main target species more readily than those using more species- and size-specific gears. Therefore, trammel net fisheries could more easily adapt to the effects of climate change on fish distribution than gill net or longline fisheries.

An ecosystem-based assessment of the Korean blue crab trammel net fishery in the Yellow Sea and management implications

Landings in the blue crab, Portunus trituberculatus, fishery in Korean waters of the Yellow Sea have declined substantially from 11,000 t in the 1980s to 2,300 t in 2004. Blue crab habitat quality in the Yellow Sea has been degraded by anthropogenic activities including sand mining, land reclamation, and coastal pollution. Various traditional management measures have been implemented, including closed seasons during spawning and size limits, but these measures alone have been unsuccessful to conserve blue crab stocks. Consequently, a total allowable catch and a stock-rebuilding program using an ecosystem-based management approach were implemented in 2003 and 2006, respectively to rebuild blue crab stocks and restore habitats. This program involved assessment of both blue crab stock status and trammel-net fishery impacts at an ecosystem-level using an ecosystem-based fisheries assessment method ( Zhang et al., 2009, 2010), which considered fishery data from catch and effort time-series, crab population biology, and ecosystem characteristics, including habitats and environmental conditions. Recent (2008) management status indices have shown significant positive change compared to conditions in 2000 with respect to sustainability of the stock and fishery and with regards to biodiversity and ecosystem habitat quality.

Size selectivity of Korean flounder ( Glyptocephalus stelleri) by gillnets and trammel nets using an extension of SELECT for experiments with differing mesh sizes

In 2008, Korean flounder ( Glyptocephalus stelleri) was designated by the Korean Ministry for Food, Agriculture, Forestry and Fisheries as a species for restoration in the coastal waters of Korea. However, there are currently no regulations regarding the minimum landed size of flounder, or the minimum legal mesh size used by the coastal gillnet and trammel net fisheries. Selectivity experiments for gillnets and trammel nets were conducted using commercial vessels in several areas off the eastern coast of Korea between October 2006 and June 2009. The selectivity analysis required an adaption of the SELECT method because trammel net mesh sizes were altered part-way through the experiment. Gillnet and trammel net selectivity was best described using a bi-modal selection curve. The two gears were found to have different selection curves for a given mesh size. The length of maximum retention was similar for the two gears, but the selection curve of the trammel net was substantially wider than that of the gillnet. For the largest size of mesh used in the commercial fishery (8.5 cm stretched), 50% relative retention was estimated to occur at a length of 20.4 cm for the trammel net, and 22.5 cm for the gillnet. In comparison, the length of 50% maturity for this species is approximately 26 cm. As there are currently no size restrictions on landings of Korean flounder, these results suggest that use of these gears may pose a serious threat to local stocks of this species.

An Assessment of the Spiny Lobster Panulirus homarus Fishery in Oman — Another Decline in the Western Indian Ocean?

The scalloped spiny lobster Panulirus homarus supports numerous traditional diving,trap and trammel net fisheries in the Western Indian Ocean. Commercial catches made in 2003to 2005 in the Arabian Sea region of Oman (a coastline of ~1100 km, comprising Dhofar andAl-Wusta) were sampled for length and sex composition, and female reproductive condition.On average, lobsters caught in Dhofar were larger than in Al-Wusta, and length-based estimatesof growth parameters, size at maturity and natural mortality were consistent with fast-growingtropical lobsters. Compared to historical data from Oman dating from the late-1980’s (with peakcatches of >2000 t/yr) and mid-1990’s, recent landings, average lobster size and size at maturitywere considerably lower. A length-based cohort analysis suggested that fishing mortality does notimpact on lobsters 80 mm. The spawningbiomass ratio (Bsp/B) of

Elasmobranch bycatch in a trammel net fishery in the Portuguese west coast

Abstract Elasmobranchs are an important bycatch component in Portuguese artisanal fisheries, but there are no management or monitoring programs in place yet. This study analysed elasmobranch catches in a Portuguese west coast trammel net fishery. Thirty-seven sampling surveys were conducted aboard commercial fishing vessels on a seasonal basis between October 2004 and August 2005. A total of 11 elasmobranch species were caught (seven Rajiformes, two Torpediniformes and two Carchariniformes), representing 4% of total catches and 15% of total weight. Raja clavata was the most important species in number (7.4 ind 10,000 m −1 of net) and Raja undulata the most important in weight (8512.4 g 10,000 m −1 of net). Discards represented 7.8% of total catch in weight, but 24.8% in number of specimens caught. Seven elasmobranch species were commonly discarded. There was a marked seasonality in elasmobranch catches, with lowest species richness registered in spring (four species) and highest in autumn (11 species). For the most abundant species depth range preferences were assessed.

Catches of target species and bycatches of an artisanal fishery: The case study of a trammel net fishery in the Portuguese coast

In this work we studied an artisanal fishery targeting mainly soles ( Solea senegalensis and Solea solea) and cuttlefish ( Sepia officinalis) with trammel nets. Catches of target species, bycatches and discards were studied from October 2004 until August 2005 in the west coast of Portugal. A total of 37 sampling surveys onboard commercial fishing vessels were conducted. The number and weight of individuals of all species caught were registered, as well as the amount discarded and retained by fishermen. A total of 112 species were identified: Scomber japonicus, Chelidonichthys obscura and Callioynimus lyra were the most discarded fishes, corresponding to 28% in weight and 36% in number of discards. Discards represented 22%, in weight, of the total catches, while the amount retained by fishermen for consumption or direct selling was 12%, also in weight. The overall estimate of the annual discards value due to this fishery was ca. 170 tonnes, comprising near 45 fishing vessels. Inconsistencies between the catches recorded onboard and the official landings were detected for the main target species. Some bycatches of species under recovery plans were also found.

Catch per unit effort of coastal prawn trammel net fishery in Izmir Bay, Aegean Sea

Catch per unit effort (CPUE) data from the coastal trammel prawn net fishery was estimated from fishery-dependent surveys in Izmir Bay (Aegean Sea) during the 2004-2005 fishing season. Catches were sampled from 18 boats for a total of 170 landings, 60 landings from April to July (first period) and 110 landings from August to November (second period). The mean CPUEs were 2.56 ± 0.39 kg. 10 net unit-1 in the first period and 2.29 ± 0.20 kg. 10 net unit-1 in the second period, and not significantly different between periods (P>0.05).

Bycatch Reduction in Trammel Net Fishery for Prawn (Melicertus kerathurus) by Using Guarding Net in İzmir Bay on Aegean Coast of Turkey

In this study, by-catch reduction in trammel nets used for prawn fishery in Izmir Bay was studied between May and October 2003. In shrimp trammel net fishery catching of discard species is an important problem due to a series of reasons such as disentangle species causes extra work for fishermen and decreases the labour life of nets. A Guarding net was attached to lead line of traditional shrimp trammel net as a modification to minimize this problem. Control Nets (CNet), originated from traditional commercial nets, and were compared against two groups of Experimental nets (Exp1 and Exp2) in two periods. In the first period of the study Exp1 net group caught 36.46% fewer shrimp (Melicertus kerathurus) in comparison to CNet. In addition to this, 35.62% crab (Goneplax rhomboides), 40.71% mantis shrimp (Squila mantis) and 44.77% purple dye murex (Bolinus brandaris) were caught fewer than those in CNet net group. In the second period of the study Exp2 net group were put into the trial. In this period, Exp1 net group compared with CNet net group caught less shrimp (8.09%), crab (66.15%), mantis shrimp (26.79%) and purple dye murex (32.25%). Exp2 net group caught fewer shrimp (0.99%), crab (50.63%), mantis shrimp (17.33%) and purple dye murex (25.92%) than CNet net group. This study indicates that the net modification was capable of reducing bycatch. Raising of trammel net above the lead line from muddy ground prevent discarding of demersal species from net. Therefore the height of guarding net is an important criterion for bycatch reduction.

The first record of the Jinga shrimp, Metapenaeus affinis (H. Milne Edwards, 1837) (Decapoda, Penaeidae) from the Mediterranean Sea

Of the nearly 600 alien marine species of Metazoa that have been recorded in the Mediterranean Sea, two-thirds have established durable populations of ever larger range (Galil, 2008a, b). The majority of the aliens originate from the Indo Pacific region and have entered the Mediterranean through the Suez Canal (Ery threan aliens). Nine species of alien penaeid prawns occur in the Levant Basin, eastern Mediterranean, all of them known from the Turkish coast: Marsupenaeus japonicus (Bate, 1888), Metapenaeus monoceros (Fabricius, 1798), Penaeus semi sulcatus De Haan, 1844, Melicertus hathor (Burkenroad, 1959), Metapenaeop sis aegyptia Galil, 1990, Metapenaeopsis mogiensis consobrina (Nobili, 1904), Metapenaeus stebbingi (Nobili, 1904), Trachysalambria palaestinensis (Steinitz, 1932), and Fenneropenaeus merguiensis (De Man, 1888) (cf. Galil, 2008a, b). All the alien penaeids but one (E merguiensis) have entered the Mediterranean through the Suez Canal. The first three species are highly valued commercially and consti tute most of the prawn catch off southeastern Turkey (Duruer et al., 2008). Extensive studies conducted in 2003-2005 of the commercial prawn trammel net fishery targeting the native Mediterranean prawn, Melicertus kerathurus (Forsk?l, 1775) in Izmir Bay, have revealed no alien prawns, either in the landed catch or in the discard (Gok?e & Metin, 2007; Akyol, 2008).

Non‐commercial invertebrate discards in an experimental trammel net fishery

Abstract Non‐commercial invertebrate discards in an experimental trammel net fishery were studied in relation to selectivity of the gear, depth, soak time and season. Forty experimental fishing trials were carried out over a 1‐year period with six combinations of small mesh (100, 120 and 140 mm) inner and large mesh (600 and 800 mm) outer panels. On average, 43.8 ± 12.2 (SD) (individuals 1000 m−1 of net) of non‐commercial invertebrates were discarded, accounting for 48% and 65% of the total catch and total discards by numbers, respectively. Within non‐commercial invertebrates discards, the six most abundant species by number were Phallusia mammillata (Cuvier) (27.5%), Cymbium olla (L.) (13.0%), Sphaerechinus granularis (Lamarck) (11.3%), Paracentrotus lividus (Lamarck) (10.9%), Astropartus mediterraneus (Risso) (8.2%) and Astropecten aranciacus (L.) (8.1%); Echinoderms (43.1%) particularly important. The highest and lowest discard ratios were found in autumn and winter, respectively. Discards generally decreased with depth, varied considerably in relation to soak time and were not related to mesh size combinations. Trammel nets seem to be the most important gear in terms of ecological impacts on benthic invertebrates compared with other coastal fishing gears and at the depths studied (15–60 m).

Fish by-catch species from coastal small-scale shrimp trammel net fishery in the Aegean Sea (Izmir Bay, Turkey)

Fish by-catch from coastal small-scale shrimp trammel net fishery was estimated from fishery-dependent surveys in Izmir Bay (Aegean Sea), Turkey, during the 2004 and 2005 fishing seasons. Sampling data were obtained from 170 observations onboard 18 commercial fishing vessels. Fifty-two species belonging to 31 families of fish, including cephalopods, constituted the by-catch. Diplodus annularis was the most abundant by-catch species followed by Loligo vulgaris > Sepia officinalis > Mullus barbatus > Boops boops > and Solea vulgaris. The estimated total ratio of fish by-catch to shrimp was 1 : 1.09 kg.

Assessment and bioeconomic analysis of the Majorca (NW Mediterranean) trammel net fishery

Trammel net is the main fishing gear used by artisanal fishers in Majorca Island (NW Mediterranean), and is representative of Mediterranean small-scale fisheries using static gear. The use of static gears close to the coast, where seasonal variability and spatial heterogeneity are high, promotes the diversification of fishing practices or metiers .W e analyze the seasonal dynamics of the nets used, target species (red and black scorpionfish, red mullet and cuttlefish) exploitation patterns and the socio-economic conditions under which this fishery takes place, based on General Fisheries Directorate daily sale records (2002-2006). Additional personal socioeconomic interviews allowed the characterization of the trammel net fishery, its fishing behaviour, cost structure and conflicts. Catch and effort data and market surveys are used to evaluate the level of exploitation of the target species and allowed describing Majorca trammel net fishery as an activity based on a sustainable resource. The main target species were found to be near their maximum sustainable yield both by means of a surplus production model (with parameters estimated by CEDA-Catch Effort Data Analysis, software) and an age structured assessment model based on yield-per-recruit analysis. Management actions currently debated by local administration and fishers focus on improving fishers' economic situation, rather than on protecting a threatened resource. One of the management actions proposed is a one day effort reduction, which was analyzed here with the help of a bioeconomic simulation model. A 15-year (2005-2020) simulation allows providing advice to local managers to focus on the commercialization aspects, in order to obtain a higher value to the fish production, rather than expecting to obtain higher profits only by a reduction of the offer.

Landed and discarded catches from commercial prawn trammel net fishery

Summary This study presents estimates of numbers and biomass of discarded and landed catches from the commercial prawn trammel net fishery in Izmir Bay. A total of 43 finfish and 29 invertebrate species were identified in catches sampled between May and October 2003, with 26 species landed by fishers and 46 species discarded. Melicertus kerathurus, Mullus barbatus and Solea solea dominated the landed catches. Average ratio of landed to discarded fish by number and weight was estimated as 1 : 2.32 and 1 : 0.77, respectively, demonstrating relatively low by-catch ratios compared to other shrimp by-catch studies. This study shows that trammel nets have a much lower discard ratio than the shrimp trawls in other parts of the world. The benefits of shifting toward passive trammel nets in coastal areas are discussed.

Trammel net catch species composition, catch rates and métiers in southern European waters: A multivariate approach

We identified and quantified the effect of season, depth, and inner and outer panel mesh size on the trammel net catch species composition and catch rates in four southern European areas (Northeast Atlantic: Basque Country, Spain; Algarve, Portugal; Gulf of Cádiz, Spain; Mediterranean: Cyclades, Greece), all of which are characterised by important trammel net fisheries. In each area, we conducted, in 1999–2000, seasonal, experimental fishing trials at various depths with trammel nets of six different inner/outer panel mesh combinations (i.e., two large outer panel meshes and three small inner panel meshes). Overall, our study covered some of the most commonly used inner panel mesh sizes, ranging from 40 to 140 mm (stretched). We analysed the species composition and catch rates of the different inner/outer panel combinations with regression, multivariate analysis (cluster analysis and multidimensional scaling) and other ‘community’ techniques (number of species, dominance curves). All our analyses indicated that the outer panel mesh sizes used in the present study did not significantly affect the catch characteristics in terms of number of species, catch rates and species composition. Multivariate analyses and seasonal dominance plots indicated that in Basque, Algarve and Cyclades waters, where sampling covered wide depth ranges, both season and depth strongly affected catch species compositions. For the Gulf of Cádiz, where sampling was restricted to depths 10–30 m, season was the only factor affecting catch species composition and thus group formation. In contrast, the inner panel mesh size did not generally affect multidimensional group formation in all areas but affected the dominance of the species caught in the Algarve and the Gulf of Cádiz. Multivariate analyses also revealed 11 different métiers (i.e., season–depth–species–inner panel mesh size combinations) in the four areas. This clearly indicated the existence of trammel net ‘hot spots’, which represent essential habitats (e.g., spawning, nursery or wintering grounds) of the life history of the targeted and associated species. The number of specimens caught declined significantly with inner panel mesh size in all areas. We attributed this to the exponential decline in abundance with size, both within- and between-species. In contrast, the number of species caught in each area was not related to the inner mesh size. This was unexpected and might be a consequence of the wide size-selective range of trammel nets.

Semi-pelagic Longline and Trammel Net Elasmobranch Catches in Southern Portugal: Catch Composition, Catch Rates and Discards

In Portugal, elasmobranch landings have decreased substantially in recent years. In this work, elasmobranch catches in experimental hake near bottom semi pelagic longlines (1997 and 1998) were compared with those of experimental trammel nets (2000). In the hake semi pelagic longline fishery, 7 elasmobranch species represented 33.4% (2185 specimens) of the total fish captures. The most abundant species were Galeus melastomus (63.3%), Etmopterus pusillus (21.7%) and Scyliorhinus canicula (14.2%). Most of these elasmobranchs (68.3% in total) were discarded. In the trammel net fishery, 16 different elasmobranch species represented 4.3% (597 specimens) of total fish catches and the most important species were Raja undulata (43.6%) and Scyliorhinus canicula (10.2%). The majority of the elasmobranchs caught with this gear had commercial value, and only 5.4% were discarded. In both fisheries, intra -specific catch rates varied with depth. Length frequency distributions for the only species with relatively high catches in both fisheries, the small-spotted catshark, showed that in general, trammel nets catch larger specimens and in a more restricted length range than longlines. Introduction Elasmobranch fishes have life strategies that make them especially vulnerable to overexploitation (Pratt and Casey, 1990; Smale and Goosen, 1999; Wintner and Cliff, 1999; Hazin et al., 2002; Coelho and Erzini, in press). Worldwide, the problem of elasmobranch overexploitation has been well documented for the case of targeted fisheries (Bonfil et al., 1990; Campbell et al., 1992; Hurley, 1998; Stevens at al., 2000) as well as those with high levels of by-catch of these species (Buencuerpo et al., 1998; Mckinnell and Seki, 1998) and for other fishing activities, such as recreational fishing (Francis, 1998). Elasmobranch fishes are a very important component of the by-catch of the artisanal fisheries in Portugal (Erzini et al., 2000; Erzini et al., 2001), but no management program has been established yet. In the past few years, catches decreased 46.3% in the Algarve (from 888.1 t in 1988 to 447.0 t in 2000) and 30.1 % at a national level (from 5634.4 t in 1997 to 3938.6 t in 2000) (DGPA, 2000). In some cases, those reductions have been very drastic and might even indicate that these species are at high risk. Such is the case of the gulper shark (Centrophorus granulosus) where there was a catch reduction of 92.3 % at a national level and 80.4 % in the Algarve, or the smooth hounds (Mustelus spp.) that registered reductions of 79.4% at a national level and 54.2% in the Algarve. The rays and skates (Raja spp.) have also suffered great reductions, namely 43.8 % at a national level 48.7 % in the Algarve.

Size selectivity of a trammel net for oval squid Sepioteuthis lessoniana

The size selectivity of a trammel net for herded oval squid Sepioteuthis lessoniana in Tateyama Bay, Chiba Prefecture, was estimated by comparison between the mantle length frequency distributions of oval squid caught by a trammel net and by a set net. The measured mesh sizes of the inner net of the trammel net and of the final section of the set net were 85.3 and 11.3 mm, respectively. In the trammel net fishery where oval squid are herded into the net, most of the oval squid are caught in the bag-shaped inner net. Hence, the logistic function was employed for the size selectivity curve of the trammel net. The ‘share each length's catch total’ (SELECT) model was implemented for the estimation of the selectivity curve. The size selectivity r(l) of the trammel net for the oval squid was expressed as a logistic function of the mantle length l: r(l)=exp (-18.57+0.88 l)/[1+exp (-18.57+0.88 l)]. From these logistic parameter estimates, the 50% selection mantle length and selection range (L75-L25) were calculated as 21.07 and 2.49 cm, respectively. The selection probability of oval squid whose mantle girth was equivalent to the mesh perimeter of the inner net was 0.09. Accordingly, oval squid of a girth smaller than the mesh perimeter were likely to pass through the mesh to escape from the net.

Characteristics of yearly fluctuation in the size of tile fish population in Wakasa Bay and its mean generation time.

Analyses were made with yearly fluctuation in tile fish population size in the fishing area of the sweeping trammel net fishery in the central region of Wakasa Bay in order to obtain its characteristics, using the data on the fishery over the period of 1961 to 1981. The estimated population size ranged from 45 to 229 tons and showed a remarkably cyclical pattern with the period of less than five years, 1961-1981. The length and market size category compositions indicated that the cyclical pattern resulted mainly from the great year classes of 1959, 1964, 1968, 1971, 1975 and 1977 and negligibly poor year classes of the other years. Consequently, it follows that the six dominant year classes mainly supported the population, and hence the fishery, in that period. The time intervals between two subsequent dominant year classes are nearly within the mean number of breeding seasons of the female fish for a situation in which fishing pressure was hardly exerted on the population.