Age, Growth and Mortality of Poor Cod (Trisopterus Minutus L.) from the Eastern Adriatic Sea

Arch. Biol. Sci., Belgrade, 67(3), 921-927, 2015 DOI:10.2298/ABS141106053S

AGE, GROWTH AND MORTALITY OF POOR COD (TRISOPTERUS MINUTUS L.) FROM THE EASTERN ADRIATIC SEA

Mate Šantić1,*, Armin Pallaoro2, Ivana Mikulandra3, Biljana Rađa1 and Ivan Jardas2

1 Department of Biology, Faculty of Natural Science and Mathematics, University of Split, Split, Croatia. 2 Institute of Oceanography and Fisheries, Split, Croatia. 3 Elementary School Vodice, Vodice, Croatia.

*Corresponding author: [email protected]

Abstract: The age, growth and mortality parameters for poor cod, Trisopterus minutus, from the eastern Adriatic were analyzed. Samples were collected monthly from January to December 2009 on a random basis, with a total of 1200 specimens analyzed (885 females and 315 males). The values of parameter b (the exponent of the arithmetic form of length-weight relationships) of males (2.97), females (3.01) and total sample (2.99) indicated isometric growth. Fish aged 1-6 years were present in the samples, including a high proportion of 2-year-old individuals. The oldest female was estimated to be 6 years old, while the oldest male was 5. The von Bertalanffy growth parameters were L¥ = 26.81, K = 0.186, and to = 1.41 for males;

L¥ = 29.5, K = 0.197, and to = 1.33 for females; L¥ = 28.76, K = 0.194, and to = 1.34 for all specimens. This study revealed that poor cod is a relatively slow-growing fish with intensive growth during the first two years of life. Total and natural mortality were Z = 1.01 year–1 and M = 0.50 year–1, respectively. The exploitation rate, E = 0.50, revealed a high fishing pressure on the stock in the studied area.

Key words: Adriatic Sea; Trisopterus minutus; age; growth; mortality

Received November 6, 2014; Revised January 15, 2015; Accepted January 16, 2015

INTRODUCTION annual landings have fluctuated between 1191 to 3534 tons from 2003 to 2005 (FAO, 2007). In the Adriatic Poor cod (Trisopterus minutus Linnaeus, 1758) is a ga- Sea, studies of age, growth, mortality and expatiation doid fish, which inhabits the Mediterranean and east- are scarce. Only Giannetti and Gramitto (1988) have ern Atlantic coast from Norway to the coast of Moroc- provided some information about the age and growth co (Svetovidov, 1986). It is common in the Adriatic Sea of T. minutus from the central Adriatic Sea. In the on sandy-muddy bottoms between depths of 40 and Mediterranean Sea, growth of this species has been 250 m (Jardas, 1996). This species is serial spawner studied by Politou and Papaconstantinou (1991) and with a prolonged spawning period. It spawns in the Ragonese and Bianchini (1998). winter-spring seasons (Jardas, 1996). It reaches first sexual maturity before the end of the first year of life Determination of age and growth rate is very (Tangerini and Arneri, 1984) The poor cod is an op- important in ichthyologic investigations, as fish portunistic predator whose diet consists of various growth is one of the main factors that determines bottom-living prey groups, with a wide range of sizes stock conditions. Parameters of length-weight re- and morphologies (Šantić et al., 2009). In the eastern lationships (LWRs) have several applications in the Adriatic Sea, T. minutus is one the main target species field of fish biology, physiology, ecology and fishery of the commercial trawl fisheries, with an annual catch assessment. Furthermore, LWRs are fundamental of about 40 tons (Jardas, 1996). In the Mediterranean, for the estimation of weight-at-age (Petrarkis and

921 922 Šantić et al.

Stergiou, 1995), production and biomass of fish For estimation of age, otoliths were removed, im- population (Anderson and Gutreuter, 1983) as well mersed in water to remove surrounding membranes, as for comparisons of fish population from different cleaned and dry-stored in small envelopes. The mas- regions (Gonçalves et al., 1997). sive and thick structure of poor cod otoliths does not allow them to be read whole. For this reason, The aim of the present work was to determine we used the “thin cross section” technique (GFCM, the age, growth and length-weight relationship as well 1982). Otoliths were fixed on small paper tags, with a as mortality and exploitation rates of poor cod. The piece of double-sided tape, the concave side facing the findings of this study are important as they can be tag, and then embedded in a mixture of paraffin and used to develop a management and protection strat- calcium oxide with a small quantity of carbon pow- egy for this species in the Adriatic Sea. der to add black color contrast. A thin section (0.25- 0.40 mm thick) was obtained by cutting the otoliths through the nucleus. Sections were placed in ethanol MATERIALS AND METHODS 75% and examined under reflected light with a ste- reomicroscope. The number of opaque and hyaline Samples of T. minutus were collected from five locali- rings was checked separately by three different read- ties in the eastern central Adriatic on the continental ers. When two readings of the same otolith resulted in shelf, mostly at depths of 90 to 120 m (Fig. 1). Fish different age estimates, the otolith was considered un- were sampled with commercial bottom-trawls using readable. Only coincident readings of 607 individuals a 22-mm-stretched-mesh size cod-end. The dura- (202 males and 405 females) were accepted. The mean tion of each haul was 2-3 h with trawling speed be- lengths of individuals assigned to each age group were used to estimate the growth parameters of the von tween 5 and 7 km/h. Monthly random samples (100 Bertalanffy equation (Ricker, 1975): specimens per month) were collected from January to December 2009, with a total of 1200 specimens mea- TL = TL ∞(1− e −k (t−to ) ) sured. Total length (TL) of all specimens was mea- sured to the nearest 0.1 cm, and body weight (W) to where TL¥ is the ultimate length that an average the nearest 0.1 g. Sex was assigned macroscopically. fish could achieve, K is the growth coefficient that

determines how fast the fish approaches TL¥, and to The relationship between weight and total length is the hypothetical age for TL = 0 cm. A maximum of fish, W = aTLb, was converted into its logarithmic likelihood test was used for comparison of growth expression: log W = log a + b log TL. Parameter b parameters for males and females. is the exponent of the arithmetic form of LWRs and Estimates of instantaneous total mortality (Z) the slope of regression line in the logarithmic form were calculated using the linearized length-converted (Froese, 2006). The value of the b exponent indicates catch curve (Pauly, 1984). Natural mortality (M) was isometric and allometric growth. The parameters a estimated using the general regression equation of and b were calculated by least-squares regression, as Pauly (1980): was the coefficient of determination (r2). Significant difference of b values from 3, which represent isomet- Log H = -0.0066 - 0.279 log L¥ + 0.6543 log K ric growth, was tested with the t-test (Pauly, 1983). + 0.4634 log T Differences in b values between the sexes were calcu- where L¥ and K are parameters of the von Bertalanffy lated by analysis of covariance (ANCOVA, P >0.01). equation. Parameter T is the annual mean water The sex ratio (males:females) for the entire sample temperature value (oC), which was 17.8 for the giv- was estimated. A chi-square test was used to detect en sampling area (Zore-Armanda et al., 1991). The differences in the sex ratio of the sampled fish. instantaneous rate of fishing mortality (F) was esti- Age, growth and mortality of poor cod 923 mated from the difference between Z and M. The nant for both sexes. In age classes from 1 to 5 years, exploitation rate (E) was determined according to the observed mean TL of females was greater than Gulland (1971): E = F/Z). that of males. The mean lengths of individuals assigned to each age class were used to fit the von Berta- lanffy growth parameters for each sex, and their data RESULTS combined (Table 2). The estimated K in the growth equation for females (K = 0.197), males (K = 0.186) Of all the fish examined (N = 1200), 885 were fe- and sex combined (K = 0.194), indicated slow growth males and 315 males. All specimens were mature. of poor cod. The growth rate of females was slightly The overall sex ratio was 2.81:1 in favor of females. A higher than that of the males, but the growth param- chi-square test revealed a significant departure from eters for males and females are not significantly dif- a 1:1 sex ratio (P >0.05). 2 ferent (max. likelihood test: L = 2.56, C 005.3 = 6.77, The length frequency distribution of males and p >0.05). According to the obtained von Bertalanffy females is shown in Fig. 2. The length frequency dis- growth equation, poor cod exhibits intensive growth tribution exhibited a mode at 13 cm. Total length of during the first two years of life. females ranged from 10.4 to 25.5 cm (mean 14.46 ± Total mortality, corresponding to the slope of the 1.89), and weight from 10.1 to 118.0 g (mean 56.2 ± descending limb of the catch curve, was Z = 1.10 year–1 13.1). Total length of males ranged from 9.2 to 20.1 cm for females, Z = 1.20 year–1 for males and Z = 1.01 year– (mean 13.3 ± 1.50), and weight from 9.7 to 112.4 g 1 for total samples. Values of natural mortality were M (mean 48.1 ± 9.26). = 0.50 year–1 for females, M = 0.49 year–1 for males and –1 Length-weight relationships were calculated sepa- M = 0.50 year for total samples. Calculation of fish- rately for males, females and for combined sexes. The ing mortality gave F = 0.60 year–1 for females, F = 0.70 year–1 for males and F = 0.51 year–1 for total samples. values of b for females (b = 3.01; t = 1.36; t0.01; tcrit = With the values of M and F known, the exploitation 2.58), males (b = 2.97; t = 1.09; t0.01; tcrit = 2.58) and ratios were computed as E = 0.54 for females, E = 0.58 both sexes combined (b = 2.99; t = 0.19; t0.01; tcrit = 2.58) indicated isometric growth. No significant dif- for males and E = 0.50 for total samples. ferences of b among the sexes were observed (AN- COVA, p >0.01). DISCUSSION Results on age determination are given in Table 1 and show that the age composition varied from 1 to In this study, the growth coefficient (K = 0.19) indicat- 6 years for females and 1 to 5 for males. According to ed that Adriatic poor cod is a relatively slow-growing the frequency of occurrence, age-class 2 was domi- fish. Namely, the area of the eastern central Adriatic

Table 1. Number (n), mean, standard deviation values (SD) of total length (in cm) and length range for T. minutus males (202 females) and females (N = 405 females) within each age class.

females males Age (years) n mean ± SD length range n mean ± SD length range 1 2 10.4 ± 0 10.4 1 9.2 ± 0 9.2 2 266 13.9 ± 1.06 11.2 – 15.8 108 12.6 ± 0.81 10.8 – 13.9 3 84 16.5 ± 0.89 15.3 – 18.5 71 14.9 ± 0.85 13.5 – 16.5 4 33 19.0 ± 0.93 17.6 – 20.1 20 17.0 ± 0.92 15.5 – 18.6 5 20 20.5 ± 0.63 19.9 – 21.9 5 19.9 ± 0.74 19.7 – 20.1 6 2 24.4 ± 0.54 23.3 – 25.5 – – – 924 Šantić et al. is one of the most oligotrophic areas of the Adriatic fish. In the central Adriatic Sea, Giannetti and Gramito Sea, which could possibly explain the slower growth (1988) as well as Politou and Papaconstantinou (1991) of poor cod. Both males and females exhibit the fast- (from the Hellenic waters) found 1- to 5-year-old in- est growth during the first two years of life, while af- dividuals. The maximum age obtained in these inves- terwards their growth slows down. A similarly slow tigation areas (Adriatic Sea, Hellenic waters, Strait of growth of poor cod (K = 0.18) was calculated by Poli- Sicily) is lower than that observed by Albert (1993), tou and Papaconstantinou (1991) in the Hellenic wa- who found that the oldest specimen in the Norwegian ters (eastern coast of Greece). On the contrary, in the waters was 8 years old. Furthermore, cool waters pro- Strait of Sicily, Ragonese and Bianchini (1998) reported duce larger, older, later-maturing individuals of species a higher growth coefficient (K = 0.46). Such differences than do warm waters (Ross 1988). Similar results are in the growth rates can result partly from the different also reported by Pallaoro (1996) comparing Adriatic techniques used but more likely reflect slight environ- saddlead bream (Oblada melanura) with population mental differences (temperature, food availability, hy- of this species in the colder seawaters. drographic nature). Poor cod is a relatively short-lived species in the Adriatic Sea. In the present study, growth In our study, although the growth parameters of analyses showed that the oldest male was 5 years old males and females were not significantly different, the and the female 6, similar to the oldest specimens noted length frequency distribution showed that females are by Ragonese and Bianchini (1998) from the Strait of slightly longer than males. These results coincide with Sicily, who described it as a small-sized and short-lived those of other authors from the Hellenic waters and

Fig 1. Study area and sampling localities of T. minutus in the eastern central Adriatic: A – near the islands of Vis and Svetac, B – south of Maslenica, C – Split Channel, D – Blitvenica fishing area, D – Jabuka Islands. Fig. 1.

1 Age, growth and mortality of poor cod 925 eastern central Adriatic (Politou and Papaconstanti- (1981), poor cod were collected during the spawning nou, 1991; Giannetti and Gramito, 1988). The differen- period (winter), when gonads are massive, which in- tial length between sexes may be related to differences creased the growth exponent (b). Tangerini and Arneri in metabolic activity, this being more marked in males, (1984) sampled fish only in May, when the same pa- with higher oxygen consumption (Pauly, 1994 a, b). rameter has relatively high values. The positively allometric growth reported by Dulčić and Kraljević (1996) The exponent of the length-weight relation- could be explained by the small sample size dominated ships for males (b = 2.97), females (b = 3.01) and by larger individuals. Values of b exponent reported sexes combined (b = 2.99) estimated in the eastern on the eastern coast of Greek waters and northern Ae- central Adriatic, showed that the growth of poor cod gean Sea (Politou and Papaconstantinou, 1991) also was isometric, meaning it is proportional in length and significantly differed from our results. These differ- weight. On the other hand, Froglia and Zoppini (1981), ences probably reflect the growth variations caused by Tangerini and Arneri (1984) and Dulčić and Kraljević environmental factors such as temperature and salinity (1996) noted positively allometric growth of poor cod in the different areas of investigation. For instance, the in the Adriatic Sea. The observed difference in the average annual sea-surface temperature of the north- Adriatic population could be due to the period of cap- ern Aegean Sea is 15-17oC with an average salinity of ture, stage of gonad maturity, length of fish and sample 33 ppt (Poulos et al., 1997), whereas the average an- size. For instance, in the work of Froglia and Zoppini nual surface temperature in the central Adriatic ranges

Fig 2. Length-frequencyFig. distribution 2. of T. minutus for males (N = 315) and females (N = 885) in the east-central Adriatic Sea.

Table 2. The Von Bertalanffy growth parameters for T. minutus from the eastern central Adriatic

2 Sex L¥ ±SE K ±SE to ±SE r n males 26.81 3.23 0.186 0.043 - 1.41 1.10 0.8951 202 females 29.50 4.11 0.197 0.030 - 1.33 0.90 0.8570 405 both sexes 28.76 3.98 0.194 0.033 - 1.34 0.78 0.8641 607

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