by J. A. A. PEREZ
Centro de Ciências Tecnológicas da Terra e do Mar (CTTMar),
Universidade do Vale do Itajaí (UNIVALI). Cx. Postal 360, Itajaí, Santa Catarina, CEP 88.302-202, Brasil (e-mail: angel@mbox1.univali.rct-sc.br)
Summary
The accretive growth of the squid pen or gladius, can be reconstructed by the interpretation of periodically depopsited growth increments. Due to overgrowth on the posterior end, early gladius increments become undistinct and the whole structure cannot be used to determine total age. However, because there is a strong correlation between gladius length and mantle length, gladius growth is suitable for reconstructing Individual Growth Histories; a powerful tool to address aspects of squid population ecology. The technique was adapted to the ommastrephid Illex illecebrosus, and growth variability during the early life inshore migration was reconstructed in order to address the effects of seasonal, geographical and oceanographic environmental gradients experienced by young squid.
Introduction
During the last 20 years, our understanding about squid populations has greatly improved from the development of ageing techniques using the squid statoliths (Jackson 1995; Arkhipkin and Perez 1998). More recently, the squid pen or gladius, has also been used for growth studies and has proved an excelent tool for reconstruction of Individual Growth Histories (Bizikov, 1991;
Arkhipkin and Perez, 1998). This paper describes the gladius technique and outlines some of its applications on the short-finned squid, Illex illecebrosus, early life ecology (Perez et al.,1996;
Perez and O´Dor, 1998).
Materials and methods The gladius technique
The gladius grows by the accretion of ventral
layers of chitin from the fins towards the head.
During periods of low growth the layers crowd together forming a distinct growth mark, resembling the gladius anterior border (Fig. 1).
Because early marks become indistinct due to overgrowth at the anterior end, total age cannot be estimated from the gladius structure. Nevertheless since there is a strong relationship between gladius length and mantle length, the gladius is suitable for reconstructing realistic Individual Growth Histories (IGH) (Bizikov, 1991; Perez et al., 1996). In fresh or formalin preserved gladii, growth marks are readily observed on the dorsal surface, under low magnification, and growth histories are reconstructed either by (a) measuring consecutive growth increments (Perez et al., 1996), or (b) by dividing a fixed gladius length interval by the number of marks counted within
88 Sesión 2: Estudios sobre edad y crecimiento de calamares y peces (Bizikov, 1991). Both techniques have been
successfully applied for the Northwest Atlantic short-finned squid, Illex illecebrosus, in which gladius increments, by comparison with statolith ring counts, were validated as daily deposited. In addition, IGHs reconstructed for captive squids, revealed that the gladius growth is sensitive to temperature variations and feeding events (Perez et al., 1996).
Fig. 1. Increments on the gladius dorsal surface of Illex illecebrosus.
The Illex problem
The early life of Illex illecebrosus in the Northwest Atlantic is characterized by an inshore movement towards the adult coastal feeding areas. During this movement, offspring experience an important food and temperature gradient established by the Gulf stream/ Slope water front system. Because spawning is extended through most of the year and recruitment may occur in different areas, similar gradients are further established seasonally and geographically. The effect of these gradients on juvenile squid growth was addressed from gladius reconstructed IGHs of individuals collected across the oceanographic front during 8 surveys conducted between 1979 and 1989. These surveys covered three geographic areas, different seasons in the northernmost area, and different positions
across the Gulf stream/ Slope water front. Gladius growth of 1,406 juvenile Illex were reconstructed and had their "recent growth" (growth during the last 15 days) tested for geographic, seasonal and oceanographic effects using an ANCOVA with repeated measurements, and the gladius length as the covariant (Perez et al.,1998).
Results and discussion
It has been hypothesized that growth and survival is maximized in the northern areas (off Nova Scotia and Newfoundland) where biological production is high when compared to warmer and food limited southern environments.
Recent growth of squids collected off Canada was significantly higher than those of squids collected off Florida and in an intermediate area, confirming the previously stated hypothesis. The same approach was conducted to test for differences in recent growth of individuals recruiting off Canada in spring, when production is high but water temperature is low, and squids recruiting later in the summer, under higher temperatures but lower food densities. Again recent growth was considerably higher in April, suggesting a close relationship between growth and food abundance (Perez, 1994). Finally recent growth improved significantly as juveniles were sampled (a) closer to the shelf, (b) away from the warm and food limited Gulf Stream waters, and (c) closer to cooler and enriched coastal waters and the shelf/slope front. These results revealed the importance of inshore migration as a strategy to attain required juvenile growth (Perez and O'Dor, 1998). The application of the gladius technique in I. illecebrosus, indicated that gladius growth is sensitive enough to evaluate ecological factors affecting growth and survival during early life, allowing distinctions to be made about the uncertain squid recruitment process (Arkhipkin and Perez, 1998). This technique can also be useful to address important population questions of Illex argentinus, a species with a complex population structure and an important target of the INIDEP-JICA Project.
Fig. 2. Results of the ANCOVA on recent gladius growth in 3 geographical areas (A) and three months of recruitment off Canada (B). Different letters indicate significantly different means.
References
Arkhipkin, A. I. & Perez, J. A. A. 1991. Life history reconstruction. In: Rodhouse, P. G.;
Dawe, E. G.; O'Dor, R. K. (Eds.). Squid Recruitment Dynamics. Fishery Technical Paper 376, FAO, Rome, p.157-180.
Bizikov, V. A. 1991. A new metod of squid age determination using the gladius. In P. Jereb S.
Ragonese & S.V. Boletzky, squids age determinations using the statoliths, p.p. 39-51.
Proceedings of the International Workshop, 9-14 October 1989, Istituto di Tecnologia della Pesca e del Pescato, N.T.R.- I.T.T.P. Special Publ. No 1, Mazzara del Vallo, Scilly, Italy. p.
39-51
Jackson, G. D. 1995. Application and future potential of statolith increment analysis in squids and sepiolids. Can. J. Fish. Aquat. Sci.
Perez, J. A. A. 1994. The eary life history of the short-fined squid Illex illecebrous (Cephalopoda:
Ommastrephidade) as reconstruted from the gladius structure. Ph.D. Thesis, Dalhouse University. Halifax, Canada, 150 pp.
Perez, J. A. A. & O'Dor, R. K. 1998. The impact of environmental gradients on the early life inshore migration of the short-finned squid Illex illecebrosus. South African Journal of Marine Science. (in press)
Perez, J. A. A., O' Dor, R., P., Beck, P. G. &
Dawe, E.G.1996. Evaluation of gladius structure for age and growth studies of the short-fined squid Illex illecebrous(Teuthoidea:
Ommastrephidade ) Can. J. Fish. Aquat. Sci., 53: 2837-2846
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