Age and Growth of the Squid Loligo Vulgaris from Portuguese Waters

Home , European squid, Loliginidae, Loligo

Not to be cited without prior reference to the authors

ICES 1996 Annual Science Conference C.M.19961K: 14 Shellfish Committee

AGE AND GROWTH OF THE SQUID LOLIGO VULGARIS FROM PORTUGUESE WATERS

Ana Moreno, JoITo M.F. Pereira and Manuela Morais da Cunha

IPIMAR Avenida de Brasflia, 1400 Lisboa, Portugal

ABSTRACT

Commercial sampies ofLoligo vlilgaris, from northwest Portuguese waters (leES Statistical squares 06EO to 09EO), were collected monthly, from January 1993 to • March 1994. The age and growth of L. vulgaris was analysed based on statolith ring counting, assuming that growth increments are formed on a daily basis. Results show that squid were recruited to the fishery with an age of about 5 months and mantle lengths from 60mm. The growth of L. vulgaris after 5 months was apower function in males and S-shaped in females. Instantaneous relative growth rates of 20%MUmonth for males and 17%ML/month for females were calculated. However the variability of individual growth rate was high. Some differences were found in growth form in squid of different broods and specimens hatched in some periods showed higher relative growth rates, although these differences were not statistically significant. The minimum age at maturity is 6 months for males and 7 months for females. Mature females showed significantly slower growth than immature. I.

INTRODUCTION

Loligo vlilgaris Lamarck, 1798 is the most commercia11y important squid in Portuguese waters, in particular in the northwest coast where its main fishery is located (Cunha and Moreno, 1994). In Portuguese waters squid are mature at an average length of 193mm in males and 242mm in females and the spawning season is in autumn and winter (Moreno et a1., 1994). Indirect methods, generally used to study growth of fish populations, based on the modal progression of length distribution with time, alone, are not considered the most appropriate to apply to cephalopod populations (Caddy, 1991). Cephalopods, particularly loliginids, have a short life span and fast growth, that can be influenced by external factors such as temperature, salinity and food availability (Forsythe and van Heukelem, 1987). As a consequence of their protracted spawning period (Moreno et al., 1994) squid • of distinct broods are submitted to different environmental conditions during the same phase oftheir life cycle depending on their hatching season, which contributes to the high variability that can be observed in individual growth rates. Thus, length modes do not necessarily correspond to age groups. Based on the analysis oflength-frequency data, maximum longevity ofL. vlilgaris was supposed to be two to three years by Tinbergen and Verwey (1945) and two to two and a halfyears by Mangold-Wirz (1963), although growth studies based on live rearing experiments and on statolith ring counting revealed higher growth rates and a life span of ca. 1 year. To study the age and growth of L. vlllgaris living in the northwest Portuguese waters, growth increments were counted in statoliths of squids caught monthly by the • commercial fishery.

MATERIAL AND METHODS

L. vlllgaris sampies from the trawl and seine fishery in ICES Statistical squares 06EO, 07EO, 08EO and 09EO were collected monthly from January 1993 to March 1994, comprising 561 males, 496 fe males and 52 specimens ofunknown sex a11 measured (ML)

2 and assigned a maturity stage, after Boyle and Ngoile (1993). statoliths of254 males, 252 females and 13 specimens ofunknown sex were removed and preserved in 95% alcohoI. Totallength ofstatoliths (SL) was measured before mounting and gnnding, following the methodology used by Rocha (1994). Statoliths of 83 males and 67 females were considered in good condition for growth increment counting, an average of 3 statoliths of males and females per size c1ass of 10mm. Counting of increments was done with a light microscope (objective mag.=20x) and an image analysis system (final magnification 1000x), from the hatching ring (HR) towards the tip of the rostrum (TR) (Fig.l). "Increment zones" and "white zones" were measured, and increments in each "white zone" were estimated from the bordering "increment zones". Statoliths with more than • 20% of "white zones" were rejected.

•

Figure 1 - Statolith of a L. vlilgaris immature male (ML=43mm).

Growth increments were assumed to forin on a daily oasis (number of increments = age of specimen), a periodicity already validated for other loliginids and ommastrephids, such as LoliolliS noctillica and Loligo chinensis (Jackson, 1990), lllex

3 illecebroslls (Dawe et al., 1985; Hurley et al., 1985) and Todarodes pacijiclls (Nakamura and Sakurai, 1991). Hatching dates were back-calculated. Relationships between statolith length (SL) and mantle length (ML), and statolith length and age were evaluated. b Xb Linear (Y=a+bX), power (Y=aX ) and exponential (Y=a e ) growth models were fitted to length and number ofincrements data for both sexes, immature (maturity stages 1,2 and 3) and mature (maturity stages 4 and 5) specimens, and specimens of different age groups (hatched in summer 1992, autumn 1992, winter 1993 and spring 1993). The model with the highest correlation coefficient (r) was chosen to describe growth. Differences in fitness (r) were tested and differences in growth were compared by analysis of covariance (Zar, 1984). The instantaneous growth coeficient (G): •

In A1L - In ML G =------2 , after Forsythe and van Heukelem (1987) (1)

where ML = mantle length, NI = number of increments in the statolith, A1L1 = mantle

length at t1 (minimum NI) and ML2 = mantle length at t2 (maximum NI), was used to calculate the instantaneous relative growth rates (% increase in length per day), by multiplying G by 100. The growth model proposed by Schnute (1981), was also adjusted to mantle length and number of increments data, for males and females. It includes numerous historical models, such as asymptotic, linear, quadratic, or exponential growth, as special cases according to different intercept (a) and slope (b) conditions. The general model (a~O and • b ~O) is represented by the following equation:

(2)

where in our case y is the mantle length (ML), t is the number of increments (days), t1 and

t2 are the limits of t, chosen to be t}=150 and t2=390 increments (growth between 5 and

13 months), Y} and Y2 are the mantle length at t} and t2 , respectively.

4 The parameters of the growth model, a ,b, Yl and Y2' were estimated numerically by the simplex iterative method ("Statistica" software package).

RESULTS

Statolith growth

The relationship between the statolith length (SL) and mantle length (ML) is described by linear equations, when ML is logarithmically transformed, indicating that • statolith growth is negatively allometric to mantle length in both sexes. Mantle length increases faster than statolith length, with no significant differences between sexes (P>0.05) (Fig.2):

Males and females SL = -0.878+0.516 In ML, r=0.843, N=532

3 E 2.5 S -5 2 Clc: .!e 1.5 -5 B oS CI) 0.5 0 3 4 5 6 7 • In ML(mm) Figure 2 - Relationship between statolith length and In mantle length in L. vulgaris.

The relationship between statolith length (SL) and the number of increments (NI) is ofthe same nature (Fig.3), and similarly no significant differences were found between sexes (P>0.05). The relationship for males and females together is described by the equation:

Males and females SL = -3.545+0.956 In NI, r2=0.581, N=150

5 Thc correlation coefficient is low, thus, in this case, statolith length is a poor predictor of age of L. vulgaris.

3 "T"'""------, 2.5 2

O-J-----r--...... ----r-.....-----,--...... ----! 4.5 5 5.5 6 6.5 In Number of increments Figure 3 - L. vlilgaris statolith growth with age. •

Age and growth by statolith increment cOll1zting

In the studied area, squid were observed to recruit to the fishery with an age of about 5 months and mantle lengths ofca. 60-70mm. Tbe largest fe male found (288mm) was 317 days oId and the largest male (485mm) had 405 days. Backcalculated hatching dates of squid sampIed (FigA) show that hatching occurs throughout the year. Differences in the number of hatchings per month does not mean monthly hatching frequencies for the population, since sampIes don't comprise a regular set of Iengths and maturity stages per month, but gives an indication of possible periods • of higher hatching rates in Iate autumn and in late winter and spring.

'0 15- '5 CT '" 'l5 10 . li; .a E :J z 5'

0- , .. Jan Apr Jul Oct Jan Apr Jul Oct I 1992 1993 I Hatching month Figure 4 - Hatching month of L. vlilgaris sampIed.

6 For males and females ofdifferent broods (each considered as hatched within a 90 days period), best fitted growth functions and instantaneous relative growth rates (between 5 and 12 months) were estimated (Table 1). Best fitted growth models varied between hatching seasons. Summer and winter hatchers presented exponential growth, for autumn hatchers no significant differences were found between the fit of a linear or apower growth model and for spring hatchers no significant differences were found between the fit of a linear, power or exponential growth model in both sexes.

Table 1 - Growth equations and instantaneous relative growth rates (G) for L. vulgaris hatched in different periods.

Hatching period N Growth equation c2 G • (%MUmonth) Summer 1992 Males 11 ML = 22.64 e O.OO7NI 0.808 22.2 Females 8 ML = 23.66 eO.OO7NI 0.870 21.0 Autumn 1992 Males 18 ML = -198.73 + 1.54 NI 0.754 34.5 Females 16 ML = -100.82 + 1.11 NI 0.501 21.6

Winter 1993 Males 12 ML = 13.54 e o.olNI 0.712 28.5 Females 9 ML =13.39 e O.OINI 0.716 28.5 Spring 1993 Males 20 ML =-87.14 + 1.00 NI 0.436 20.9 Females 13 ML = 23.91 e O.OO8NI 0.797 24.0

Males hatched during spring show slower growth (30mmlmonth) as well as • females hatched during summer (32mmlmonth), representing ca. 21 % of increase in mantle length per month, in both sexes. Faster growth rates were observed in males hatched in autumn and females hatched in winter (ca. 51mmlmonth), indicating 34.5% and 28.5% of increase in mantle length in males and females, respectively. These differences were, however, not significant (P>0.05). The age at maturity is highly variable as can be seen in figure 5. Ten month old immature males (stages 1,2 and 3), as well as 6 month old fully mature males were observed. This wide range can also be found in females, but with a slightly lower overlap than in males. In general, all specimens younger than 6 months were immature. Males mature earlier than females (males from 186 days and females from 206 days) and from the age

7 ,------

of 11 months a11 individuals were found to have reached fu11 maturity. This can also be seen from the percentage of mature males and females per age classes of 30 days, depicted in figure 6.

• • ·AlIlll'IIgll 0 Oll 2 • 2 • ....~ Vl 3 -; 3 • >. .-;::.... 4 • 4 • ::s ' ... 5 5 . ~ .' ~ • • m Malu .' m F,malu • 100 200 300 400 100 200 300 400 Age (days) Figure 5 - Age and maturation. i = immature (1,2 and 3), m = mature (4 and 5). •

_Males rz:lFemales 100 •..••••••••••••••••••

75 ••..•••..••••.•••.

50 •.•••••••••.

I I' I 4 5 6 7 8 9 10 11 12 13 Age(month) Figure 6 - Proportion of mature males and females per age class. • Best fitted growth models of immature and mature squid are displayed in figure 7 and table 2. No significant differences were found in the growth of immature and mature males (P>0.05). Immature males presented linear growth and no significant differences were observed between power und exponential growth correlations in mature males. Immature females display exponential growth und in mature females the variability is so high that a11 growth models applied showed non-significant fit. Mature females, however, showed significantly (P

8 Table 2 - Growth equations and instantaneous relative growth rates (G) estimated for immature and mature squid. N Growth equation r G (%MUmonth) Males Immature 46 ML =-87.59 + 0.94 NI 0.546 20.3 Mature 37 ML =51.5 e O.OOSNI 0.490 15.3

Females Immature 45 ML =26.04 e 0.OO7NI 0.477 21.0 Mature 21 ML =31.76 NI 0.283 0.122 3.4

Instantaneous relative growth rates calculated for immature and mature squid of ages between 6 and 11 months in males and between 7 and 10 months in females (age interval ofcoexisting immature and mature specimens), gives an indication ofthe slightly • lower increase in length ofmature males in relation to immature (not significant) and the very low increase in length of the mature females.

700 400 600 Males Females / - Immature / 300 - Immature 500 ~ / o• • ~/ • •• E 400 -.- Mature • E -.- Mature & L ... -r • .." • .5. .." .5. ---~...... J • 200 '" 300 • _-: ...J ~ 0 :::t :::t .... ---. .~ 0 . 200 .." ,/ -- ." 100 - • o... 100 / 0 0 100 200 300 400 500 0 100 200 300 400 Number ollncrements Number of Increments

Figure 7 - Growth ofL. vulgaris immature and mature males and females.

• The adjustment ofseveral types ofgrOWth models to length and age data for squid after 5 months of age (Table 3), showed a best fit to a power model in males and a linear model in females (Fig.8), however an exponential model for males and a power model for females didn't both show significant differences (P>O.05) to the best fitted models. No significant difference was found (P>O.05) in the growth between the male and female populations (differences were found with P

9 individual growth rates. For exarnple, at 9 months old, males vary between 141 and 362mm ML and females between 143 and 273mm ML. Instantaneous relative growth rates (0) of 20%MUmonth for males and 17%ML/month for females, were estimated from the best fitted growth models.

Table 3 - Orowth models adjusted to mantie 1ength (ML) and number of increments (NI) data.

Type of Model Model Equation r2 Males Linear ML =-156.4 + 1.30NI 0.636 Power ML =0.006 NI 1.85 0.644 Exponential ML =32.9 e O.OO6Nl 0.638 Females Linear ML =-78.6 + 0.99NI 0.491 Power ML =0.080 NI 1.39 0.482 • Exponential ML =49.0 e O.OO5NI 0.444

600 400 500 Males Females 300 . e- 400 . e- s 300 . S 200 -' -':; :; 200 .. 100 100 0 0 0 100 200 300 400 500 100 150 200 250 300 350 400 Number of increments Number of increments

Figure 8 - Orowth of males and females after five months old (best fitted model power far males and linear for females). •

The estimated life span would be ca. 15.2 months for males and 13.9 months for females, to attain the maximum size faund in Portuguese waters (492 for males and 334 for females after Moreno et al., 1994). The maximum age observed in the present study, however, was 13.9 months in a mature male with 399mm ML and 13.1 months in a mature female with 230mm ML. The instantaneous relative growth rate (0) decreases gradually in both sexes with age, being slightly higher für fcmales at 5 münths and higher für males, after that age

10 (Fig.9). The decrease in G is more pronounced until 9 months and more evident in females than in males. From that age the decrease in G is similar in both sexes.

40 -y------...., --Males --Females ~30 -o= .§ 20 ...:l ::E ~ 10 o O+----,-----r----,r----.,----i 4 6 8 10 12 14 Age (months)

Figure 9 - Variation of instantaneous relative growth rates (G) ofL. vulgaris at monthly • intervals.

No significant differences were found (ANCOVA test of equality of slopes, p>0.05) in the growth ofsquid hatched in different seasons of the year. Differences in the growth of immature and mature males were also not significant (ANCOVA test of equality of slopes, p>0.05). It was however observed that mature females have significantly slower growth than immature and display differences in the growth form. Thus, a more complex growth model, that could account for those differences, might better explain the female population growth. The Schnute (1981) growth model was thus adjusted to female length and age data. The best fitted growth model to females between 5 and 13 months is an S-shaped curve • (Fig.l0) described by the equation:

1 - e -0.08 (NI-ISO) ] - 9.~1 ML = 73.18-9.51 + (243.29-9.51 - 73.18-9.51 ) 1 [ _ e -0.08 (390-150) (r= 0.54)

The inflexion point, calculated from Schnute (1981), is the pair of values (261 days, 190mm) and 243mm is the asymptotic mantle length.

11 350

300 0 0 0 250 00 E.s 200 ..J :E 150 100 50 0 100 150 200 250 300 350 400 Number of increments

Figure 10 - General growth model for L. vlilgaris female population.

The schnute model applied to males, results in a growth curve very similar to a power model, the best fitted model reported before. •

DISCUSSION

The negative allometry observed in the growth of the statolith with ML, seems to be common in loliginid species and was previously reported for L. vlilgaris living in the west Saharan shelf (Arkhipkin, 1995) and for the c10se related species, L. forbesi, in the northeast Atlantic (Collins et al., 1995). Individual growth rates were found to be highly variable, especially in males, since spawning occurs throughout the year (Moreno et ai., 1994) and individuals experience different environmental conditions depending on the hatching period. This variability in • individual growth rates was already observed for L. vlilgaris by Natsukari and Komine (1992), Bettencourt (1994), Rocha (1994) and Arkhipkin (1995) and for L. forbesi by Collins et al. (1995). It was observed that females hatched in colder seasons (winter and spring) presented higher growth rates, after 5 months of age, than animals hatched in warmer seasons. In males there was no evidence of a relation of growth rates to colder or warmer seasons. Although differences in growth between animals of distinct broods were not statistically significant, the results for females agree with growth studies of L. vlilgaris in Galician waters (Rocha, 1994) and are contradictory with studies made in other squid

12 species, in which higher temperature produces faster growth in cultured L. forbesi, especially during the early exponential growth phase (Forsythe and Hanlon, 1989) and wild populations of P/zotololigo edlilis, L. forbesi and [ilex argentiizus hatched during warmer seasons have higher growth rates (Natsukari et al., 1988; Collins et al., 1995; Rodhouse and Hatfield, 1990). Tbe first mature males appeared earlier than mature females and in both sexes part of the population matures much younger than the rest. These findings are in agreement with the literature, despite some variation in reports of age at first maturitY. Arkhipkin (1985) presents older ages at first maturity for both males (250 days) and females (285 days) of L. vlilgaris in Saharan shelf and Bettencourt (1994) and Rocha (1994) both indicate younger ages at first maturity off south Portugal and Galician waters. Those • results however, may be accounted for by the great variation encountered hetween individuals. Altematively, there may be regional or time related variations. Tbe fact that full maturity is reached this early in life (6-7 months old), and at least 50% of squid are mature at 8 months, could indicate that the majority ofthe population spawn and die young, having a life span shorter than the maximum observed (13-14 months), hut it can also be an indication that spawning lasts for several months in both sexes. Signs of an extended spawning period were observed in both sexes ofL. vlilgaris by Rocha and Guerra (1995), but it is not unreasonable to think that at least part of the population have a life span of less than 1 year. Similar growth rates were found between population(s) living off northwest Portugal, off south Portugal (Bettencourt, 1994) and in the Saharan shelf (Arkipkin, 1995). Off northwest Portugal growth rates of males and females didn't differ significantly, similarly to what was observed in southem waters (Bettencourt, 1994) and in the Mediterranean (Natsukari and Komine, 1995). However, sexual dimorphism in mantle length is typical in L. vulgaris (Moreno et al., 1994, Coelho et al. , 1994, Guerra et al., 1994), so the larger size attained by males in relation to females, could be explained if only males continue to grow at a similar rate, after reaching maturity. Tbe fact that no differences were found in the growth of immature and mature males, and females grew very slowly after reaching maturity, may be an indication of the same thing. Thus, the differences in maximum size between sexes could result from a diversion in females from somatic to gonadal growth as maturation increases. Thc early growth of L. vlilgaris, verified by some authors in cuIturcd specimens,

13 is slow following an exponential funtion (Turk er al., 1986; Forsythe and Hanlon, 1989). After the first months growth is faster (verified by statolith ring counting). following a power function (Arkipkin, 1995; Bettencourt, 1994). The sampling range in the present study covers the second growth phase and the relationship between mantle length and number ofincrements suggests, in fact, that males growth after 5 months follows apower function. In females the best fit was a linear function but not significantly different from the fit of apower funtion. The variance explained by these models, due to the high individual variability. was however low and other growth models (e.g. exponential) also presented a reasonable fit. A bettercorrelation between mantle length and number ofincrements was obtained for groups of specimens hatched in different periods than for the whole sampie, due to the lower individual variability within each group. leading to the conclusion that environmental conditions must indeed playa strong effect on the growth of squid. • The best growth model found for male L. vlilgaris was a non-asymptotic model, but although asymptotic growth functions seem inappropriate to describe individual cephalopod growth, (Forsythe and van Heukelem, 1987; Forsythe and Hanlon, 1989), the best fit found for female growth was indeed an asymptotic model. The explanation for this may be the existence of slow and fast growing females and a slower growth of each individual at maturity, resulting in a clear decrease in the growth rate of the population after the age when most part of specimens are already mature, which nevertheless does not imply that each female in the population grows towards an asymptotic size. Statolith preparation and counting of increments is an extremely time-consuming technique. Attempts to establish a good relationship between age and vririables easier to obtain, such as mantle length or statolith length resulted in poor correlation coefficients, thus revealing the weakness ofmantle length/age or statolith length/age keys. Therefore. • despite its difficulties, increment reading in statoliths seems to be the only adequate tool for studies of age and growth of wild populations of L. vlilgaris. The results ofthis study should be accepted with caution as they were based on the assumption of"1 increment =1 day". This hypothesis was previously confmned for other cephalopod species. and sounds reasonable for L. vltlgaris. but it remains to be validated.

14 Acknowledgements We are grateful to Cristina Castro for all the efforts in mounting and grinding statoliths and to Pedro da Concei~äo for his assistance in the biological sampling and extraction of statoliths. We also benefitted from the help of Ivone Figueiredo whose advice in the application of growth models is heartily acknowledged. This work was funded under EC Contract No AIRl-CT92-0573.

REFERENCES

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