Growth rate estimation of Hexaplex trunculus based on mark/recapture experiments in the Ria Formosa lagoon

ABSTRACT This research reports growth rates estimates for Hexaplex (Trunculariopsis) trunculus (Gastropoda: Muricidae) from mark/recapture experiments carried on the outside in the Ria Formosa lagoon (Algarve coast, southern Portugal). A total of 726 specimens (shell extent and total weight ranging between 2065-5836 mm and 086-1989 g respectively) were marked with Dymo tape tags and released into a fish tillage earth pond. During the marking proces no adverse events on the whelks' health and behavior or immediate postmarking mortality were exposeed Periodical recapture operations were undertaken using a traditional fishing gear designated locally as "wallet-line" and by means of hand gathering by scuba divers. A total of 170 whelks were caught, with a total of 216 recaptures (shell extent and total weight ranging between 3622-6597 mm and 442-2735 g respectively), thus corresponding to a recapture rate of 298% Despite the los of individual tag, all remaining tags were intact and easily readable. Marked individuals at handed low monthly growth rates, in bounds of shell length (1.0 mm/month or 23%/month) shell perimeter (20 mm/month or 31%/month) and total weight (07 g/month or 108%/month) which were highly variable between individuals and higher in smaller specimens. Data were used to estimate the von Bertalanffy expansion parameters (length and weight) (K= 041 [L.sub.[infinity]] = 8276 [W.sub[infinity]] = 4997 and [tsub0] = 005) The expansion rate of T. trunculus was compared with be the effects obtained in similar studies with other gastropod species to evaluate its potential for molluscan aquaculture.

lock opener WORDS: Hexaplex (Trunculariopsis) trunculus, Gastropoda, Muricidae, mark/recapture, growing rate

INTRODUCTION

Marking rules have been readily applied to several species of gastropods, mainly because of the neighborhood of an external shell upon which marks or tags can be attached with little or no adverse consequences to the animals (Jones 1979 Gosselin 1993) of that kind experiments with gastropods have been used in diverse emblems of studies, to assess move (Eversole & Anderson 1988, Himmelman 1988) fishing rate (Hancock 1963) field of attraction and effective fishing area of baited traps (Himmelman 1988 McQuinn et al. 1988 Sainte-Marie 1991) species' behavior towards baited traps (Ito et al. 1980) population size and mortality (Hancock 1963) and growing rate (Hancock 1963, Eversole & Anderson 1988)

Various marking techniques have been used in gastropods, of the like kind as painting the shell spire with quick drying paints (Hancock & Urquhart 1959 Hancock 1963 Jone 1979 Ito et al. 1980) applying color codfished nail polish to the shell (Laxton 1970 Gosselin 1993) attaching rubber bands or nylon lines with tags around the shell (Appeldoorn 1988 Himmelman 1988 McQuinn et al. 1988 Sainte-Marie 1991) gluing a variety of tags to the shell (Hancock & Urquhart 1959 Smith 1987 Eversole & Anderson 1988 Amos & Purcell 2003) scrubbing the shell lip unrestrained from the periostracal layer (Hancock & Urquhart 1959 Hancock 1963) drilling and wiring the shell (Laxton 1970 Weil & Laughlin 1984 Kideys & Nash 1993 Kideys 1994) chemically marking the shell and operculum with fluorescent coloring liquors (Kideys & Nash 1993) and using underwater metal detectors to recapture specimens marked with aluminum tags (Crowe et al. 2001)

The muricid gastropod Hexaplex (Trunculariopsis) trunculus (Linnaeus, 1758) is a public inhabitant of the subtidal and intertidal areas of the Ria Formosa lagoon (Algarve coast, southern Portugal), where it is make submissiveed to a locally important artisanal fishery. This activity is traditionally undertaken the one and the other by manual harvesting during depressed tide and with an illegal and artisanal fishing gear known as "wallet-line," on the contrary more recently this species has also been caught by the agency of scuba divers operating illegally inside the lagoon. Furthermore, because of a growing demand for gastropods in the seafood market, the commercial value of T trunculus in Portugal has increased markedly in novel years (reaching prices of 10 [euro]--15/kg for first sale), which has been generating high expectations in bourns of its potential as a fresh species for molluscan aquaculture.

Integrated in a general inquiry on the biology, ecology and fishery of T trunculus in the Ria Formosa lagoon, this work aimed to assess the growing rate of this species through mark/recapture experiments in a fish agriculture earth pond. To the authors' knowledge, this was the first experiment of this kind at any time made with T. trunculus and the first development data (growth rates and extension parameters) available for this species. Moreover, the information gathered in this application of mind will be most useful for assessing the potential of T trunculus for molluscan aquaculture, as well as for establishing adequate management measures for this artisanal fishery.

MATERIALS AND METHODS

Mark/Recapture Experiments

Marking experiments were carried without with commercial samples of T trunculus from the Ria Formosa lagoon (Algarve coast, southern Portugal). Prior to marking, colonizing algae and/ or encrusting organisms (mainly polychaetes) were remov from the shell with a hard brush and a small area of the largest whorl of the shell was carefully smoothed with sandstone and clean dried with absorbent paper. Subsequently the specimens were tagged with Dymo tape with sum of two units alphanumeric characters. The margins of these tags were make an incision in with scissors, ensuring a tag of [approximately equal to] 6 x 4 mm size and facilitating adherence to the circulared and irregular surface of the gastropod shell. The tags were fixed with cyanoacrylate gelatine and covered with epoxy gelatine (Fig. 1A), to diminish abrasion of the tag caused through the burrowing movements of the whelks into the sediment and the adjustment of encrusting organisms over the tags, which could jeopardize mark retention and readability. The tags were generally fixed between the sum of two units most recently deposited growth bands of the last whorl of the shell (Fig. 1A), to debar eventual dropping of excessive gelatine from the shell onto the yielding body of the gastropods. Finally, after allowing the gelatine to dry for approximately half hour, marked individuals were rinsed in seawater to avoid potential contamination vexed questions provoked by glue residues, and maintained overnight in an aquarium with running seawater, where they were examined for general condition (comparison of behavior between marked and unmarked specimens).