• Abstract and Figures
• Public Full-text

BULLETIN OF MARINE SCIENCE, 71(2): 711–724, 2002 RECRUITMENT, GROWTH AND REPRODUCTION IN TODAROPSIS EBLANAE (BALL, 1841), IN THE AREA FISHED BY FRENCH ATLANTIC TRAWLERS J. P. Robin, V. Denis, J. Royer and L. Challier ABSTRACT Short-finned squid landed by the French fishery are by-catch of trawlers operating in the Northern Bay of Biscay and Southern Celtic sea. In the period November 1997–June 1999 samples of commercial specimen were collected monthly. A total 1065 Todaropsis eblanae were analysed for biological parameters. In a subsample of animals age was determined using statoliths. Mature animals are observed almost all year round, however juvenile recruitment occurred mainly in winter and sexual maturation increased in spring. Size at sexual maturity (DML50) was 16.5 cm in females and 13.5 cm in males. Length frequencies showed that the range of exploited stages was 9–29 cm in females and 8–22 cm in males. Statolith analysis indicated that youngest recruits were about 4 mo old and that growth was faster in females than in males (3.41 and 1.86 cm mo−1, respectively). Sex-related differences in growth and size at maturity are in agreement with previously published data from the Galician coast. Todaropsis eblanae (Ball, 1841) is an oceanic squid, which lives near the bottom on the shelf break. In the Mediterranean, it has been observed at depths between 200 and 600 m in the Western area (Quetglas et al., 2000) and in shallower waters in the Central and Eastern areas (Belcari and Sartor, 1993; Tursi and D’Onghia, 1992). In the Atlantic, south- east populations of this species were considered by Roeleveld et al. (1992) as an indicator of the necto-benthic community in the upper slope (300–800 m) off Africa whereas the species occurs on the continental shelf to the west and south of Ireland (Lordan et al., 1998). In spite of a deeper distribution than Loliginids, T. eblanae is said to have life- traits and behavior more similar to neritic squids than to oceanic ones (Clarke, 1966). Geographically, T. eblanae is widely distributed from eastern Atlantic to Mediterranean sea between 60°N and 36°S (Roper et al., 1984). The species was also recorded in South- west Indian Ocean (Nesis, 1979) and in Australian waters (Lu, 1982). T. eblanae is, together with Illex coindettii, the only Ommastrephid squid exploited by the French Fishery. This resource is mainly a by-catch for trawlers operating in the Northern Bay of Biscay and Celtic Sea. It is only a minor component of the catch which is still discarded in areas of low abundance like in the English Channel. The two species are not distinguished by fishermen and landings are not split into commercial categories. Little published information exists on T. eblanae. In European waters studies carried out to describe exploited stages concern Galician waters (Gonzales et al., 1994) and Scot- tish waters (Hastie et al., 1994). This study presents results on the Northeast Atlantic population exploited by French trawlers. The study of the proportion of this species in the landings and investigations into its population biology were based on the sampling of commercial landings in Le Guilvinec (Southwest Brittany) in the period November 1997–June 1999. In addition, the spatial distribution was derived from fishery statistics. 711 712 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 2, 2002 Figure 1. Location of the fishing harbour where biological samples were collected and ICES statistical divisions fished by French trawlers (white zone). MATERIALS AND METHODS Biological samples of landings of the trawl fishery were collected in Le Guilvinec fish-market (Fig. 1). Geographic origin of each catch was not recorded but during the study period 93% of short finned squid landed by Le Guilvinec trawlers came from two ICES divisions (Fig. 1-8A,1-7H). From November 1997 until June 1999 a 40-kg box of unsorted short-finned squid was collected monthly. Fishermen store the catch on ice and samples were kept frozen from transport until pro- cessing in the University laboratory. In Caen, species were determined and basic biological data recorded (i.e., dorsal mantle length, body weight, sex, maturity stage -according to Lipinski (1979) modified by Gonzales and Guerra (1996), total reproductive system weight, separated gonad weight and digestive gland weight). A total of 1065 T. eblanae were analysed during the sampling period. Reproduction timing was analysed with temporal variations of both gonado-somatic index (GSI) and maturity stages, histological observations having shown that GSI should not be used alone (Lipinski and Underhill, 1995). The gonado-somatic index (Mangold-Wirz, 1963) is: GSI = 100 × GW/(BW − GW) where GW is gonad weight (ovary or testis) and BW is total body weight. Wb aL Length-weight relationships were described by the power model: ROBIN ET AL.: RECRUITMENT, GROWTH AND REPRODUCTION IN TODAROPSIS EBLANAE 713 W = aLb where W is body weight, L is DML, a and b are the regression coefficients. Length at sexual maturity was estimated via the DML50 parameter (length at which 50% of the individuals are maturity stages 4, 5 and 6). This parameter was derived from the fitting of a Logistic curve to the frequencies of mature animals per length class (DML) (Coelho et al., 1994): 1 P i = abL 1+ e−()+ i where Pi is the percentage of mature individuals in length class Li, a and b are the regression coef- ficients and DML50 = −a/b. In a preliminary step, an estimation of growth was sought by studying mean length progression, which provides underestimates of the phenomenon (Caddy, 1991). Regression lines were fitted between average DML and time in males and females. A sub-sample of T. eblanae was selected for the analysis of age and growth during a recruitment period (November 1997–February 1998). In a preliminary stage, 57 statoliths of specimens ranging from 8 to 27 cm (27 females and 30 males) were ground and polished. Statolith preparation tech- niques followed a protocol named by Dawe and Natsukari (1991) “method of Arkhipkin and Villanueva” in which both sides of the statolith are ground, starting with the concave side. The main difference with Arkhipkin’s protocol is that statoliths were mounted with a heat-melting resin (Crystal Bond™) with which it was easier to invert. Grinding papers had grain sizes ranging from 3 to 0.3 mm. Increments counts often differ between light microscopy and scanning electron mi- croscopy (Lipinski and Durholtz, 1994). In this case, increments were counted in light microscopy using the T.N.P.C.® software from the nucleus to the dome edge (Fig. 2). This analysis was based on the hypothesis of daily deposition of statolith increments which had not been tested in T. eblanae but which was validated in Alloteuthis subulata (Lipinski, 1986) Idiosepius pygmaeus (Jackson, 1987) Todarodes pacificus (Nakamura and Sakurai, 1990) and in Loligo vulgaris (Natsukari and Komine, 1992). Fisheries statistics for French trawlers were obtained from the official database held by Centre Administratif des Affaires Maritimes. Monthly short-finned squid landings per ICES rectangle in the period January 1989–December 1998 were included in the Caen Cephalopods GIS Application (Pierce et al., 1998). RESULTS PROPORTION OF T. EBLANAE IN LE GUILVINEC LANDINGS.—During the study period T. eblanae was observed in all samples, although it was generally a minor component of short- finned landings (about 30%, Fig. 3). The two Ommastraephid species seemed to be ex- ploited simultaneously and not in a sequence as described in English Channel loliginids. Changes in species proportions showed that the percentage of T. eblanae was minimal in December 1997 and maximal in September 1998. Todaropsis dominated during 4 mo in summer 1998. In the light of the species life-cycle, unbalanced proportions in these 2 mo were likely a consequence of lags in the recruitment or the spawning of these two species. SEX RATIO.—Out of 1065 squid collected, 610 were females and 455 were males, which corresponds to an overall ratio of 0.75. Sex ratio in monthly samples was not signifi- cantly different from the 1:1 equilibrium except in summer 1998 and in April 1999 (Fig. 2). The unbalanced situation observed in March 1998 (68% of males) do not represent a 714 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 2, 2002 Figure 2. Statolith of a male Todaropsis eblanae; DML = 18 cm; Increment number = 177. ROBIN ET AL.: RECRUITMENT, GROWTH AND REPRODUCTION IN TODAROPSIS EBLANAE 715 Figure 3. Monthly proportion (bars) of Todaropsis eblanae in Le Guilvinec short-finned squid landings and monthly sex ratio (males:females) (line). Monthly sex ratio significantly different from 1 are indicated with white circles (χ2 test P < 0.05). significant departure from the equilibrium but rather a statistical error related to a small sample size (n = 19 in this month). SPATIAL DISTRIBUTION.—Abundance indices in Ommastrephid squid were derived from French trawlers CPUE (Fig. 4). Although fishery records are not species specific, this series of maps underlines that in the area fished by French trawlers short-finned squid are mainly observed at the shelf edge. Month-to-month variations suggest that stock spread is higher in late spring than in winter (when the distribution looks more patchy). The spread of Todaropsis and Illex in the Bay of Biscay and Celtic sea in spring is consistent with the lack of genetic differences between Irish and Spanish samples (Dillane et al., 2000). LENGTH-FREQUENCIES.—T. eblanae length-frequencies at monthly intervals were plot- ted in Figure 5. The largest specimen was a 29 cm female, although most individuals ranged between 9 cm and 25 cm in females and between 8 cm and 22 cm in males.

Load more

  14

Copy Link