Ageing seabreams: A comparative study between scales and otoliths
Introduction
Seabreams (Sparidae) are found in coastal waters world-wide and sustain important recreational and commercial fisheries (Fischer et al., 1987). This family can be found in a wide variety of marine habitats, from rocky to sand bottoms, at depths between 0 to 500 m, although they are usually more common at less than 150 m deep. In Southern Portugal, seabreams are an abundant group found on the continental shelf, lagoons and estuaries, with more than 20 species regularly caught (Gonçalves, 2000, Gomes et al., 2001, Sousa et al., 2005, Ribeiro et al., 2006).
Age information is important as it forms the basis for the calculations of growth and mortality rates and productivity estimates (Campana, 2001), making it essential for fisheries management (Casselman, 1987, Cailliet et al., 2001).
One of the main problems facing age and growth estimates is the selection of the most suitable structure to age the fish. Scales have been used widely for age estimation however, the use of scales has been criticized mainly because the ages of older fish are frequently underestimated (Beamish and McFarlane, 1983, Carlander, 1987). Otolith age determination is thought to be more accurate because otoliths have a higher priority in utilization of calcium (Carlander, 1987). Furthermore, unlike scales otoliths continue to grow as the fish ages (Beamish and McFarlane, 1983, Casselman, 1987).
The objective of this study is to compare the use of two different structures, otoliths and scales, for the study of age and growth, for seven sparid fishes, bogue, Boops boops (Linnaeus, 1758), common two-banded seabream, Diplodus vulgaris (Geoffroy Saint-Hilaire, 1817), white seabream, Diplodus sargus (Linnaeus, 1758), the striped seabream, Lithognathus mormyrus (Linnaeus, 1758), axillary seabream, Pagellus acarne (Risso, 1827), common pandora, Pagellus erythrinus (Linnaeus, 1758), and black seabream, Spondyliosoma cantharus (Linnaeus, 1758).
Section snippets
Sampling
Samples were obtained from gear selectivity studies conducted in south Portugal, complemented by market sampling and beach seining for juveniles inside a temperate coastal lagoon (Ria Formosa, south Portugal). Procedures and sampling details are described in Erzini et al., 1996, Erzini et al., 1998, Erzini et al., 2003 and Gonçalves (2000). Data collection during gear selectivity studies was made from 1992 to 1999 using longline and gillnet, and beach seining was used for data collection from
Results
Overall, otoliths and scales of all seven species showed a regular growth pattern, alternating opaque and translucent increments. Marginal increment analysis suggesting an annual growth increment formation, validating the use of otoliths and scales for age determination (Fig. 1). The peak of the increment varied through the year for each different species. For the same individuals scales provided, on average, more growth increments per years than otoliths in the following species: D. sargus, D.
Discussion
Although data from the otoliths has already been published for some of the species, namely the bogue (Monteiro et al., 2006), the common two-banded seabream (Gonçalves et al., 2003) and the axillary seabream (Coelho et al., 2005), the von Bertalanffy growth function (VBGF) was fitted again in order to have exactly the same procedures for both sets of data (otoliths and scales). The small discrepancies found between the results previously published and the ones in the present study are due to
General conclusions
Growth parameters were successfully estimated for all seven species. In general the parameters estimated based on data for all individuals combined were reasonable. The consequence of the lack of juveniles in the samples of some of the species is reflected in the relatively large absolute values of the parameter t0. Overall, one can say that otoliths are generally better structures to age these sparid fishes because they presented smaller standard deviations and higher R2. Scales may be used as
Acknowledgments
This work was funded in part by the Commission of the European Communities DG XIV through the project ref. 98/082: Fisheries biology and assessment of demersal species (Sparidae) from the South of Portugal. The authors would also like to thank two anonymous reviewers that made valuable comments and suggestions.
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