Exploitation of the volute snail Zidona dufresnei in Argentine waters, Southwestern Atlantic Ocean.

ABSTRACT Zidona dufresnei (fine snail) is a common gastropod species in the Southwestern Atlantic Ocean, which has been exported to Asian countries for human consumption since 1988. Landings of Z. dufresnei increased from 1974 to a maximum record of 1,300 mt (whole shelled animal) in 1997, followed by a steady decline through 2002. No resource management effort have been applied to this species since the beginning of its catch history, about 30 y ago. Information provided in this work, together with previous studies on the reproductive biology and population dynamics of Z. dufresnei, will contribute to establish effective precautionary polices to manage this resource. We suggest that Z. dufresnei is in a phase of over-exploitation since 1988 until present, we propose a rational management as size selectivity, with a minimum capture size of 16 cm, closure of the fishery during the reproductive season, from September to December and to rotate the fishing areas. This study provides historical and updated information as an attempt to implement corrective measures towards a better management of the fishery on the Argentine Continental Shelf.

KEY WORDS: volutes, marine snail, Zidona dufresnei, coastal fishery, Argentine Sea 

INTRODUCTION

Marine gastropods represent approximately 2% of the molluscs fished in the world, and several gastropod species, such as Haliotis spp., Strombus spp., Busycon spp. and Concholepas concholepas have high economic value in international markets and play substantial social roles in small-scale artisanal fisheries (Leiva & Castilla 2002).

Resource sustainability has been difficult to achieve when incomes are growing. The number of overfished populations, and the indirect effects of fisheries on marine ecosystems, indicates that management has failed to achieve a principal goal, sustainability (Botsford et al. 1997). This situation has been promoted through a fishing behavior involving successive exploitation cycles, as well as the replacement of depleted stocks or fishing areas by new ones. As a result, various marine gastropods were incorporated as a new fishery resource. However, many of these show serious problems of overexploitation because of their high economic value and excessive capture (Tegner 1989, Castilla 1996, Ponce-Diaz et al. 1998, Hobday et al. 2001), resulting in some cases in the collapse of the fishery. The economic and social consequences caused by the depletion of marine gastropod stocks promoted the development of new management policies focusing on biologic and economic sustainability (Prince et al. 1998, Castilla 1997, 1999, 2000).

In recent years Zidona dufresnei (Donovan, 1823) became a valuable resource in Argentina. This species inhabits the western coast of the Southern Atlantic Ocean, between 25 and 60 m depth on sandy bottoms from Rio de Janeiro, Brazil (22[degrees]S) to Patagonian waters of San Matfas Gulf, Argentina (42[degrees]S) (Kaiser 1977). Snails are exported as fresh meal or canned products to different markets, mainly to Asian countries, with only a small proportion being used in the domestic market. In particular, Z. dufresnei has been exploited by the Argentinean and Uruguayan fleets for the last 30 y and is one of the few volutids of great commercial value occurring in the fishing area common to both countries. Only the large muscular foot, representing 60% of the whole body mass without shell, is used for consumption.

Previous studies on Z. dufresnei have focused on its taxonomy (Clench & Turner 1964) and anatomy (Novelli & Novelli 1982, Aycaguer 2002). During the reproductive cycle of Z dufresnei, emission of gametes occur in two pulses during the austral spring and summer (October to March), with two peaks of major activity in October and January to February (Gimenez & Penchaszadeh 2002). Although a small proportion of snails attain sexual maturity at a minimum shell length of 12.8 cm, 50% of the population becomes sexually mature at a size of 15.7 cm and 15 cm for females and males, respectively (Gimenez & Penchaszadeh 2003). In this species, maximum lifespan is 18 y and individuals are sexually mature at an age of 8-9 y. (Gimenez et al. 2004).

This article summarizes information on the fishery of Z. dufresnei in Argentina and provides historical data and updated catch reports related to biologic parameters and propose a rational management of the resource.

MATERIAL AND METHODS

The study was carried out between January 1999 and December 2000 off the Mar del Plata coast in Buenos Aires Province, Argentina (Fig. 1) at depths from 40-60 m on sandy and mixed shell-sand bottoms. The data used in this study were obtained from commercial fishing at the Mar del Plata harbor. The bottom trawling fleet was composed of 25 vessels ranging from 20-26.5 m long equipped with and 450-650 HP engines, and a trawl net of 42 mm mesh size.

[FIGURE 1 OMITTED]

Historical catch and landing data were obtained from FAOSTAT Fishery Data (FAO 2004). Every month, one box randomly chosen from landings at Mar del Plata harbor containing about 50 individuals of Z. dufresnei was sampled. The shell length of each snail was measured to the nearest nun with a vernier caliper. The proportion of Z. dufresnei males and females was compared using the [chi square] test (Sokal 1998). The sizes of individuals caught between 1999 and 2000 were compared using the Kruskal-Wallis test.

To evaluate the importance of the snail fishing areas: A, Querandi (37[degrees]10'S); B, Mar del Plata (38[degrees]20'S); and C, Quequen (39[degrees]10'S) (Fig. 1), we analyzed the daily weight of the total catch of Z dufresnei (SAG y P, 2002) per boat to obtain the catch per unit of effort (CPUE, mt/boat/day). The CPUE of Z dufresnei among areas and between years was compared using 2-way ANOVA. All landings took place at the Mar del Plata harbor. Monthly catches among and within the three fishing sites during 2000 were compared using repeated-measures analysis of variance (ANOVA). Prior to analysis, variables were tested for normality and homoscedasticity. When necessary, Tukey test was used for a posteriori comparisons.

The faunal composition of the by-catch was studied from material obtained on board of a "Capitan Canepa" vessel (INIDEP) on a research cruise performed on October 1999 using a trawl net of 2 cm mesh.

RESULTS

Historical Data

Although in Argentina snail fishery started in 1974 (FAO, 2004), it was established as a commercially relevant fishing resource in 1988 (Lasta et al. 1998). Landings of Z dufresnei increased from 1974 onwards to reach a maximum record of 1300 mt in 1997, followed by a steady decrease through 2002, when landing values dropped to half as compared with 1997 (Fig. 2).

[FIGURE 2 OMITTED]

The size structure of Z. dufresnei individuals sampled in 1999 (n = 508) and 2000 (n = 543) from landings at the Mar del Plata harbor and captured in the Mar del Plata fishing area (area B), is shown in Figure 3. The analysis of the sizes of Z. dufresnei revealed the presence of individuals below the size of first maturity during all months between 1999 and 2000 (Fig. 4). The percentages of juveniles were 38.6% and 37% for 1999 and 2000, respectively. There were monthly fluctuations in the frequency of juveniles, and peaks equal to or higher than 65% were observed in March and May of the two studied years.

[FIGURES 3-4 OMITTED]

There were no annual variations in the proportion between the foot weight and size of snails. The proportion of sexes calculated from 1,051 snails landed in the Mar del Plata area was 1:1 (513 males, 538 females; [chi square] test, P > 0.05).

Species Composition of Catches

Ninety per cent of volutids captured in the Mar del Plata fishing area were Z. dufresnei, and the remainder was Adelomelon beckii. The most conspicuous by-catch species of the Z. dufresnei fishery is shown in Table 1. By-catch represented 5% of the total capture.

Landings Seasonality

During the study period (1999 to 2000) (Fig. 5), significant differences were found between months (1-way ANOVA, [F.sub.[alpha] = 0-.05, 3, 20] = 3.3027, P < 0.05), where landings decreased during the winter months of 1999 and 2000. (Tukey test P < 0.05).

[FIGURE 5 OMITTED]

The daily fishing activities allowed comparison of the CPUE of Z. dufresnei obtained from sampling areas A (Querandi), B (Mar del Plata) and C (Quequen) between 1999 and 2000. No significance differences were found in the CPUE attributable to year-fishing area interaction (2-way ANOVA, [F.sub.[alpha] = 0.05, 2, 508] = 0.1938; P > 0.05) (Fig. 6). There were significant differences in the CPUE among areas (2-way ANOVA, [F.sub.[alpha] = 0.05, 2, 508] = 13.9184 P < 0.001). Area C showed the highest CPUE values (Tukey test P < 0.05). No significant differences were found between years (2-way ANOVA, [F.sub.[alpha] = 0.05, 1, 508] = 1.5962; P > 0.05).

[FIGURE 6 OMITTED]

Monthly catches from the three fishing areas during 2000 are shown in Figure 7. There were no significant differences among areas (Repeated measures ANOVA, [F.sub.[alpha] = 0.05, 8, 116] = 1.38798; P > 0.05), but significant differences were found in landings attributable to month-fishing area interaction (repeated measure, ANOVA, [F.sub.[alpha] = 0.05, 4, 58] = 55.39524; P < 0.001). A Seasonal pattern was observed in all fishing areas.

[FIGURE 7 OMITTED]

DISCUSSION

Landings of Z. dufresnei in the Mar del Plata area showed a marked seasonality (Fig. 5). During the winter time, the drop in landings reflects lower catches because the harsh climatic conditions, which reduce the access of the coastal fleet to the fisheries areas (personal communication from fishermen, Mar del Plata). In addition, the low landings in winter are caused by the closure of the Z. dufresnei fishery after the recorded peaks of PSP toxins taking place in autumn (Carreto et al. 1996).

At present, Z. dufresnei is the most important snail fishery in Argentina because it is located in areas of easy access where it is found in high densities and has a better palatability in comparison with other co-occurring volutid species.

Data suggest that the fishery of Z. dufresnei showed the same temporal exploitation phases proposed by Castilla and Fernandez (1998) for the gastropod Concholepas concholepas in Chile. Such exploitation phases are as follows: (1) "Initial exploitation phase," characterized by relatively low and constant landings, there are no major foreign market openings and an absence of management frameworks. In C. concholepas this phase extended between 1960 and 1975 and in Z. dufresnei between 1974 and 1984. (2) "Expansive extraction phase," characterized by an increasing demand from foreign markets (e.g., Asia and the United States), which increased export of shellfish. In C. concholepas this phase extended between 1976 and 1980 and in Z. dufresnei between 1984 and 1988. (3) "Over-exploitation phase," characterized by strong foreign market forces, exponential increase in unit prices and easy access to stocks, with the concurrent increase in catch volumes. In C. concholepas this phase extended between 1979 and 1989, and in Z. dufresnei between 1988 until present. (4) "Closure phase," characterized by overfished or severely depleted populations determining multimonth or annual fishery closures (Defeo et al. 1993, Castilla 1997). Fishery closures were thus used to evaluate the capacity for recovery of depleted populations under absence of fishing (Castilla & Defeo 2001). This phase occurred for C. concholepas, from 1989 to 1992. (5) "Stabilization of extraction and institutionalization phase." The information collected on the previous phase improves the quality of management guidelines. Operational management instruments based on area-specific management plans (minimum legal sizes, fishery gear restrictions, total catch levels per fisher and catch per fishing ground) are implemented in Chile (Castilla et al. 1998). C. concholepas has been in this phase since 1992. (6) "Mature and consolidation phase," characterized by sustainable exploitation over time through effective resource management. The muricid C. concholepas fishery is close to attaining this phase (Castilla 1999, 2000).

This information suggests that Z dufresnei would be following the same pattern of phases as C. concholepas. Presently Z. dufresnei seem to be in the over-exploitation phase. The fact that this phase was longer for Z. dufresnei than for C. concholepas would affect the rate of resource depletion.

Until present, no studies have been conducted to determine appropriate mesh sizes of nets and to evaluate or improve fishing selectivity for gastropods in the Southwestern Atlantic. Therefore, the question about maximum individual production in function of capture: "could selection be optimized?" still remains unanswered. As with most marine benthic resources in Latin American countries, Z. dufresnei still continues in the "over-exploitation phase."

Over-exploitation can also be detected through the analysis of sizes from captured specimens. The frequency of individuals with large body sizes decreased, whereas those with intermediate sizes increased. When the size at first sexual maturity (Gimenez & Penchaszadeh 2003) was compared with the size distribution of collected snails, more than one third of the latter was below the size of sexual maturity. Fluctuations observed in Figure 4 indicated that during summer months when fishing pressure was heavier, the proportion of individuals below the size of sexual maturity was greater than in the rest of the year.

The analysis of landing data indicates that Mar del Plata (area B) is the most important fishing area with the heaviest exploitation. Information on the accompanying species is useful to assess further changes in the local community, thereby reducing potential impacts on the ecosystem. Indirect trophic (food web) interactions induced by fishery removals represent a second class of important indirect effects of fishing. (Botsford et al. 1997)

Even when mussels (Mytilus edulis platensis), oysters (Ostrea puelchana) and scallops (Aequipecten tehuelchus) are species of commercial importance, only the first develop in high densities near the study area. Although by-catch represents 5% of the total capture, further studies regarding the by-catch community composition must be performed to assess changes in the ecosystem and impacts associated by the bottom trawling used in these fisheries.

Guidelines that could be adopted for the management of this small-scale fishery are the rotation of exploited areas, together with size-selective fishing (from the size at first sexual maturity onwards) and fishing coordinated with the reproductive cycle.

Z. dufresnei is vulnerable to over-exploitation because of slow growth (Gimenez et al. 2004), late attainment of sexual maturity (Gimenez & Penchaszadeh 2003), low dispersal capacity caused by the direct development of a small number of embryos within egg-capsules attached to hard substrates (Penchaszadeh & de Mahieu 1976).

We propose a minimum capture size of 16 cm for Z. dufresnei, taken into account that the size at first maturity is 15 cm for males and 15.7 in females (Gimenez & Penchaszadeh 2003). We recommend a fishery ban for Z. dufresnei during part of the reproductive season, from September to December (Gimenez & Penchaszadeh 2002). All facts mentioned earlier in this article address the necessity and urgency of establishing polices to regulate the minimum capture size and to protect fishing areas of Z. dufresnei.

ACKNOWLEDGMENTS

The authors thank SENASA and Fishermen of Mar del Plata harbor. J.G. and G.B. were supported by a fellowship from CONICET (Argentina). This work was supported by Agencia de Promocion Cientifica PICT-10975; PICT-14419,UBACYT-X316.

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JULIANA GIMENEZ, (1,2) * MARIO LASTA, (3) GREGORIO BIGATTI (1,2) AND PABLO E. PENCHASZADEH (1,2)

(1) CONICET. Lab. Invertebrados, Depto. Ciencias Biologicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pab. II. C1428EHA; (2) Museo Argentino de Ciencias Naturales, Av. Angel Gallardo 470 Buenos Aires, Argentina; (3) Instituto Nacional de Investigacion y Desarrollo Pesquero INIDEP, Mar del Plata, Argentina

* Corresponding author. E-mail: [e-mail]

TABLE 1.
By-catch composition from the Zidona dufresnei
fisheries in Mar del Plata area.

Principal
Groups            Species

Echinoderms       Astropecten brasiliensis brasiliensis Muller &
                    Troschel 1842
                  Arbacia dufresnii (Blainville 1825)

Annelids          Aphrodita longicomis Kinberg 1855

Bivalves          Aequipecten tehuelchus (d'Orbigny 1846)
                  Mytilus edulis platensis d'Orbigny 1846
                  Ostrea puelchana d'Orbigny 1841

Gastropods        Adelomelon beckii (Broderip 1836)
                  Crepidula argentina Simone, Pastorino &
                    Penchaszadeh 2000

Crustaceans       Libinia spinosa Milne Edwards 1834

Fishes            Paralichtys patagonicus Jordan 1889
                  Percophis brasiliensis Quoy & Gaimard 1824
                  Squatina argentina (Marini 1930)
                  Discopyge tschudii Heckel 1846