by
M. R
ENZI, B. SANTOS, D. HERNANDEZ, R. ROTH and A. BARRAL Instituto Nacional de Investigación y Desarrollo Pesquero (INIDEP). C.C. 175,Paseo Victoria Ocampo N° 1, Mar del Plata, 7600 - Argentina.
e-mail: marenzi@inidep.edu.ar Summary
Merluccius hubbsi, or Argentine hake, is distributed throughout the Argentine Sea. Earlier research on morphometrics, age and growth, genetics and parasites, suggested the existence of at least two discrete stocks. The goal of this study was to evaluate the utility of otolith microchemistry as a tool to discriminate hake stocks. Juveniles and adults came from three regions: (i) 34°S-39°30'S, (ii) 39°30'S-44°S, and (iii) South of 44°S. Analysis was based on the composition of sagitta otoliths of twelve elements (Zn, Cu, Mn, Sr, K, Na, Ni, Ca, Mg, Li, Fe, Ba), using atomic absorption and emission spectrophotometry. Data were analized by using ANOVA/MANOVA and discriminant analysis. Significant differences in the elemental concentrations were found among hake from the three areas. Also, differences between north and south of 41°S (management units used at present) were found. Discriminant analysis was also carried out with different age groups with a low rate of successful classifications.
128 Sesión 2: Estudios sobre edad y crecimiento de calamares y peces individual fish into zone groups and age groups.
Differences between North and South of 41°S, and among the three ecosystems (34°S-39°30'S;
39°30'S-44°S; and South of 44°S), were tested.
Results and discussion
As for other species, the elements in hake sagittae constituted three distinct sets of elements:
macroelements: Ca and Ba; microelements: Na, Ni, Li, Sr, K and Mg; and trace elements: Fe, Mn, Zn and Cu.
Examination of the microchemistry for other fish species (Thresher et al., 1994; Proctor et al., 1995) appears to be comparable to its of hake, except for Ba and Ni which seem to be extremely high in our analysis.
MANOVA by locality showed differences between north and south of 41°S (***P<0.0001), and also among the three zones mentioned above (***P<0.001).
In the first case considering the two stocks complex, ANOVA of each element showed that four elements were significantly different between localities (Ca and Na, **P<0.01; Mn and Sr, *P<0.05). Considering the three zones only Sr and Ni were distinct among zones (**P<0.01).
A good separation resulted between north and south of 41°S groups with an 82% classification rate (Fig.1), based on one significant discriminant factor onto which two elements loaded significantly (0.47 for Na, and -0.47 for Mg).
However it was unexpected as these elements are
physiologically important and their concentrations in the fish are likely to be strictly controlled.
Considering the three areas significant differences were also observed, (88% of classification rate). Separation was based on two significant discriminant factors; the first one onto which Sr loaded significantly (0.81) contributed to separate 39°30'S-44°S zone from the others (Fig. 2). The data to support the environmental sensitivity of otolith composition are not abundant (Secor, 1992; Radtke et al., 1990) but to date all involved Sr, which has been reported as sensitive to changes in salinity, temperature and pH. The second discriminant factor separated samples from 34°S-39°30'S; and south of 44°S, presenting higher loadings for Mn and Ni (0.47 and -0.41, respectively)
Differences in the diet of hake distributed among the three areas may account for discriminating groups of individuals or stocks through microchemistry.
Discriminant analysis considering otolith microchemistry and age data resulted in a strong separation among 1 year group and the rest. It was also observed the homogeneity of group 2, and the overlap of 3 and 4 age groups. Separation was based on two discriminant factors, with a rate of successful classification of 57 %, owing to the large overlap among age 3 (0.81%) and 4 fish (0.54%). Differences in otolith trace element content between juveniles and adults were found.
Fig. 1: Distribution of hake from north and south of 41°S in discriminant factor space.
Fig. 2: Distribution of hake from three zones in discriminant factor space.
A relative decrease in most of the elements like Ca, Cu, K, Na, and Fe was observed. However this observation corresponds to different cohorts so ontogenetic comparisons would require more specific sampling.
Although considering this study as the first approach in hake otolith microchemistry, we may tentatively conclude that its application would be a very useful tool for discriminating Argentine hake stocks.
References
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Introduction
Long tailed hake, Macruronus magellanicus Lönnberg, 1907 (Pisces, Macruronidae) (Nelson, 1994), is a demersal-pelagic species (Angelescu
& Prenski, 1987), which is distributed in the Atlantic and Pacific oceans, surrounding South America, where it is caught from 36° to 55°S and 100 to 700 m deep.
In the Southwestern Atlantic Ocean it is distributed between 50 and 500 m deep, mostly on the continental shelf, being the highest concentrations located below 200 m, including San Matías and San Jorge gulfs (Flowers & Roa, 1975; Roa et al., 1976; Bezzi, 1984). Due to the discontinuous distribution of the species between the San Matías Gulf and the Patagonian shelf, with particular environmental conditions between each other, it was necessary to test if more than
one demographical group are here involved.
Due to the presence of juveniles younger than 1 year-old, inside the San Matías Gulf (40°50'-42°20'S and 64°-65°W) and southwards De los Estados and Tierra del Fuego Islands (55°S), raises the question whether there is only one demographical group with many spawning grounds, or if more than one stock is involved.
Looking for evidence to test these hypothesis, differences in the somatic and in the otolith growth are analyzed.
Material and methods
A total of 1595 sagittae otoliths belonging to 877 individuals from the Patagonian shelf and 718 from San Matías Gulf were collected during May 1993 and April 1994, respectively (Fig. 1).