Bathymetric Distribution and Movements of Red Mullet Mullus Surmuletus

MARINE ECOLOGY PROGRESS SERIES Vol. 166: 247-257,1998 Published May 28 Mar Ecol Prog Ser

Bathymetric distribution and movements of red mullet Mullus surmuletus

'institute of Marine Biology of Crete. PO Box 2214, GR-71003 Iraklion, Greece 'University of Crete. Department of Biology. PO Box 1470. GR-71110 Iraklion, Greece

ABSTRACT Using data from bottom-trawl surveys conducted each summer, winter and spring on the Cretan shelf from 1988 to 1991, we examined the bathymetric distribution of red mullet Mullus surmuletus.Additionally, we used data from 3 yr (1991 to 1993) of monthly sampling in the Iraklion Gulf. Depth, temperature and salinity data were combined with biological data on abundance, biomass, fish size, age, sex and maturity. The ranges of bottom depth, temperature and salinity over which red mullet is distributed were established. In general, f~shsize increased with bottom depth, and smaller individuals tended to be found in shallower and warmer water. Abundance increased in mid-shelf waters during spring, indicating a movement across the shelf towards deeper water. The latter seemed to be associated with spawning behavior. The factor controlling the timing of that movement to deep water seemed to be the maturity of individual fish.

KEYWORDS: Red mullet. Distribution . Depth selection - Temperature selection - Migration

INTRODUCTION A general trend for larger fish to occur in deeper water has long been known, especially with respect to Knowledge of the distribution and movement of an deep-sea fishes (Haedrich & Rowe 1977). The fishes of exploited stock, particularly during its reproductive the inshore zone also seem to undertake an ontoge- phase, is required for proper management of the netic migration to deeper water but there is a paucity resource (Mullen 1994, Wroblewski et al. 1995). How- of studies on the phenomenon and its causes (Mac- ever, studies on depth, temperature and salinity pref- pherson & Duarte 1991, Warburton & Blaber 1992, erences of marine fishes are widely scattered in the Blaber et al. 1995). However, there is disagreement as literature and based on few observations (Scott 1982). to the generality and the underlying cause of this phe- This is especially true for demersal species on the nomenon (Macpherson & Duarte 1991 and references Mediterranean continental shelf. Caddy (1993) hypo- therein, Swain 1993). More detailed studies on the thesized the existence of an offshore movement of causes of the ontogenetic movements have concen- older fish in several demersal species (e.g.sea breams, trated on long-lived species, especially cod and plaice common pandora, mullets, etc.), possibly contributing (Harden Jones 1968), but similar investigations are to continuing high recruitment in many areas, as well lacking for short-lived species like red mullet. Ste- as to stock recovery. The main problem in the study of fanescu et al. (1992) noted that further study of this these species is that the available data in the Medi- subject should center on the autoecology of each spe- terranean come from commercial fisheries, hence cies, as the complexity of links established by a species there is limited possibility for investigation of the with its environment will result in highly variable and relationships between biological and environmental specific adaptive responses in each case. parameters. Such investigations require long-term The red mullet Mullus surmuletus (L. 1758) is a com- research surveys. mon species on the Mediterranean shelf. It is distributed along the eastern Atlantic from the North Sea to the northern part of West Africa (Hureau 1986). In the

O Inter-Research 1998 Resale offull article not permitted 248 Mar Ecol Prog Ser 166: 247-257, 1998

Mediterranean Sea, the red mullet is subject to inten- et al. 1991): 26-70 m (stratum I). 71-150 m (stratum 11) sive fishing (Stergiou 1990, Rehones et al. 1995). and 151-350 m (stratum 111). Stratum I covered 25%, Several aspects of red mullet biology have been stratum I1 covered 24 *4, and stratum 111 covered 51 % of studied, including feeding, reproduct~on, age and the total area. Stratum I spanned a substrate covered growth (see Reiiones et al. 1995 and references there- by algae (Caulerpa prolifera) and sea grass (Posidonia in), but information on its bathymetric distribution and oceanica and Halophila stipulacea); the substrate of movements is limited to depth ranges. Hureau (1986) stratum I1 consisted mostly of mud, sand or detritus; reported that Mullus surmuletus inhabits depths less and the substrate of stratum 111 was covered for the than 100 m and Macpherson & Duarte (1991) found a most part by crinoids (mostly Leptometra phalangium). depth range of 12 to 182 m. All tows were carried out approximately in parallel In the present study, we examine the seasonal with the 100 or 200 m isobaths during daytime. No changes in the distribution pattern of red mullet off the tows were taken in water deeper than 350 m. coast of Crete (eastern Mediterranean). Research sur- The duration of each haul ranged from 50 to 90 min vey data on distribution, abundance, biomass, size, at a towing speed of 1.8 to 3 knots, depending on the age, sex and maturity of red mullet were compared depth and nature of the bottom. Gear selectivity was with depth, temperature and salinity in order to eluci- assumed to be constant because the same vessel (RV date factors (e.g. maturation) controlling the seasonal 'Philia') and fishing gear (trawl with a cod-end bag timing and directinn of fish movement. liner of 22 mm stretched mesh-size) were used in the survey. The catch from each haul was identified to species, enumerated and weighed, and the bottom depth, MATERIAL AND METHODS water temperature and salinity were measured. The door spread of the trawl net was calculated for each Sampling. The seasonal distribution of red mullet at haul based on the method of Carrothers (1980). Total various depths and within various temperature ranges area swept was calculated by multiplying the door was studied using data from 3 yr of seasonal sampling. spread by the vessel speed and the fishing time. Fish Seasonal bottom-trawl surveys were carried out on the abundance was calculated as number of fish or bio- Cretan shelf from August 1988 to April 1992. Specifi- mass per square nautical mile (n mile2). The depth of cally, one survey was conducted in summer (August to tow was determined by means of an echo-sound.er early September), one in winter (December) and one in (average depth of tow was used). Bottom temperature spring (late March to early April) of each year. Each and salinity were measured using a SEA-BIRD CTD survey comprised about 40 random, depth-stratified unit. stations, in all areas of the shelf able to be trawled Additionally, we used available data from 3 yr of (Fig. 1). Three depth strata were selected (Tsimenides monthly surveys (January 1991 to December 1993) on

-S 8 --. CRETAN SEA ,'-> 4 i ', /-

,~20\0-u- CJr .--loo- -

/---.---X S' / *' . .-'Oo Q SE#lO ZOO? -,l Fig. 1. Map of the sam.pled 20 n rn~~e. LIBYAN SEA area indicating the 100 and 200 m isobaths as well as 2 2 e" the position and d~rect~onof I I the hauls (arrows) Machias et al..Bathymetric distribut~onand movements of red mullet 249

Table l Mullus surmuletus. Female and male maturity stages based on macro- circuli in the lateral field, close spac- scopic examination of the gonads. Adapted from Macer (1974). Hilge (1977), ing of circuli followed by wider spac- N'Da & Ileniel (1993) ing in the anterior field, and consis- tency of these characteristics in at I Stage- Description I least 4 of the scales examined per fish. - Age classes were assigned accord- Females I Immature-resting Ovariessmall; less than one half of length of ventral ing to number of scale annuli, mar- cavity; pale pink to red. No granular appearance ginal-increment spacing, month of

I1 Developing Ovaries becoming larger; onehalf of length of ventral collection, and size reldtive to the cavity 01- more; plnlush or yellow. Granular appearance length frequency distribution (Ross I11Late-developing Ovdr~esvery large; pale yellow or yellow. Transparent 1988). We used May 1 as the birthdate, to running hyaline eggs based on the results of the gonad study IV Spent Ovarles flaccid; pink or red. Early stage of atresia ~ISI- and on unp.ublished data froill ichthy- ble in hyaline oocytes (white spots) oplanktonic investigations in the same Males area. A total of 1526 specimens were I Immature-resting Testesvery small; translucent or grey. Less than half of length of body cavity aged and the von Bertalanffy growth curve (VBGC) was estimated as in I1 Developing Testes becoming larger and white; more than half of length of body cavity. Lobed formation Tserpes & Tsimenides (1995). 111 Late-developing Testes very large and multilobed; creamy white or Growth curves for females and to running pinkish; free-flowingmilt males were estimated separately and 1V Spent Testes flaccid; possibility of a little residual milt compared by analysis of residual sum of squares (Chen et al. 1992). Since the 2 VBGCs were not significantly the shelf of the Iraklion Gulf (off central northern different (F= 0.015, p > 0.05) data were pooled and the Crete). These data provided complementary and tein- resulting curve was used for a deterministic aging of porally more detailed information on changes in the all fish caught, using the equation: depth distribution of red mullet. These additional data 1 L t = to--111 1-2 (1) were from a total of 19 hauls for each month (at least k [ L-1 5 hauis at each station). Each monthly survey corn- where L,is total length at age t; L, is asymptotic length; prised a trawl at each of 3 stations (1 in each of the k is growth coefficient; to is theoretical age at zero 3 strata) using the same gear and sampling method as length (Hilborn & Walters 1992). in the seasonal surveys. Data analysis. After logarithmic transformation (Mid- Biological sampling. During the 9 seasonal sur- dleton & Musick 1986, Stefanescu et al. 1992), means veys, either all specimens or a random subsample of of abundance (ind. n mile-') and biomass (g n mile-2) at least 100 individuals from each haul were mea- were calculated for each cruise (1) for each 50 m bot- sured and weighed. The following measurements tom-depth interval, and (2) per stratum (I,11, 111). Fur- were taken for each specimen: total length to the thermore, the seasonal distribution of the total abun- nearest mm, tota.1 and eviscerated mass to the nearest dance and biomass for each stratum or bottom-depth 0.1 g, and gonad mass to the nearest 0.1 mg. The sex interval were calculated by weighting the mean values and the macroscopic stage of maturity (Table 1) were with the ratio of the surface area of the stratum or also determined and a gonosomatic index calculated bottom-depth interval to the total area. gonad weight Analysis of variance showed no differences in abun- GSI = ( eviscerated bodv weiahtd dance or biomass among the 3 surveys in the same sea- transformed to number of individuals per n mile2. son, nor among the 3 strata (l,II, 111) in the same season To study growth, scales were collected from all spec- (0.177 < p 0.730; Bartlett's test, 0.101 < p < 0.562). For imens during the first year of seasonal surveys, and each season, results and conclusions were similar and from fish larger than 220 mm during subsequent sur- independent of whether survey-specific or pooled data veys. Six scales from each specimen, taken from the were used. Hence, only the latter are presented here. region under the pectoral fin, were cleaned with water, A correlation analysis (Pearson's correlation coeffi- placed on 0.3-mm-thick cellulose acetate plates and cient) was performed to determine if there were any pressed at a temperature of 105°C for 3 min. Subse- significant changes in abundance or biomass wlth bot- quently, the scales were removed and their imprints on tom depth or water temperature. The same method the plates were examined using a binocular micro- was used to test the hypothesis that fish size is depth scope. Annuli were identified using the standard crite- dependent. Fish size was also tested for any significant ria of Bagenal & Tech (1978), especially cutting-over of correlations with temperature. Geometric mean was 250 Mar Er01 Prog Ser 166 247-257, 1998

preferred for calculation of the mean fish size of each significance of temperature and bottom depth selec- sample, because the arithmetic mean is susceptible to tion. The test statistic D was defined D = maxlf(t)-g(t)l the influence of a few large specimens and does not (maximum absolute vertical distance) when f(t) and represent accurately the mode in fish size at a given g(t) were the 2 functions compared at 0.2OC (CDFs of station (Stefanescu et al. 1992). To test the hypotheses temperature) or 10 m (CDFs of depth) intervals. Signif- 'fish size is depth dependent', 'larger individuals teid icance was assessed using randomization tests (Perry & to be found in deeper water' and 'smaller fish tend to Smith 1994, Swain & Krammer 1995). be found in shallow water', correlation analysis was Standardized numbers of males and females in the 4 performed between depth and mean, minimum or different maturity stages (Table l), by age class, were maximum length (Middleton & Musick 1986, Mac- used in the spring sampling to examine differences in pherson & Duarte 1991, Stefanescu et al. 1992). The maturity among strata. Al.1 statistical inferences were same 3 variables were used in correlation analysis with based on the 0.05 significance level. respect to temperature. The distribution of different-sized fish was simpli- fied, as follows: Lengths were converted to ages using RESULTS Eq. (1).We examined relationships between red mullet density (ind. n mile-') and temperature or bottom Abundance and biomass depth usir?g cum~~.lativedistribution functions (CDFs) following Perry & Smith (1.994). The CDF (in %) for A total of 6280 ~ndividuals were analyzed. Miriiirs temperature (available temperature) or bottom depth surmuletus was distributed between depths of 28 and (available depths), f(t),was calculated for each season 310 m, between temperatures of 13.6 and 23.8OC and as follows: between salinities of 38.12 and 39.?5%0 (Table 2). Salinity showed very small variation and is llui generally considered to affect the distribution of fishes on the where I =

Table 2. Seasonal ranges of depth, temperature and salinity in which red mullet occurred in the period 1988 to 1991 and t is a level of temperature or bottom depth; Ah is the area of stratum h; nh is the number of tows in stra- Min. Max Range tum h; xh,is the bottom temperature or depth of tow i in stratum h; L is the number of strata. The CDF for red Depth (m) mullet catch, g(t),was calculated similarly: Spring Summer L nh A Winter CC--" Yhl I Temperature ("C) g(t) = loo--- I,:I ,-I where I = 1, ifxhi

100 * 4 C 1- SUMMER WINTER T 4 * SPRING

T SUMMER

Fig. 2 Mullus surmuletus. Seasonal distribution by abundance and biomass (A) !E 50 m depth ir?te:va!s [m) and (B)in the 3 strata. I: stratum I (20-70 m); 11. stratum I1 (71-150 m):111. stratum 111

Abundance W Biomass Cretan shelf (Tsimenides et al. 1991). Abun- dance and biomass were significantly nega- tively correlated to bottom depth during all seasons and positively correlated to temperature during summer (Table 3). No significant I correlations between abundance or biomass and temperature were observed during winter and spring. The lack of correlations could be 80 mainly attributed to the small temperature variation during these seasons (Table 2). 60 Relative percentage of abundance and biomass by depth interval (Fig. 2) indicated that ,o .. fish generally occurring in shallow waters increased their presence at mid-shelf depths (stratum 11) during spring. A similar change 20 was observed in the monthly collections in the --a--- Iraklion Gulf (Fig. 3). Fig. 3 shows that, after o -I - * the aforementioned change in May, abun- l23456789101112 .- Stratum l - a --.Stratum II- StXZli] dance and biomass remained low in stratum I - - - P- .= and high in stratum I' up to recruitmentduring Fig. 3. Mullus surrnuletus.Monthly distribution in the Iraklion Gulf, by Jul~-August.Thus, the existence of a sea- abundance in the 3 strata (I,11,111). Months are numbered from January sonal, depth-related movement is suggested. (1) to December (12) 252 Mar Ecol Prog Ser 166: 247-257, 1998

Table 4. Correlation between mean, minimum and maximum total length Fish size (TL) of red mullet and depth and temperature The relationships between mean total Mean TL Min. TL Max. TL length (TL) and depth were significantll- r P r P r P positive in all seasons (Table 4).The maximum TL of the samples increased with Depth Spring 0.645 0.000 0.631 0.000 0.204 0.107 depth during summer and winter. Mini- Summer 0.791 0.000 0.801 0.000 0.573 0.002 mum TL of the samples increased with Winter 0.833 0.000 0.814 0.000 0.465 0.005 depth in all seasons. The relationship Temperature between mean TL and temperature was Spring -0.215 0.108 -0.203 0.107 -0.198 negative in summer and winter, but there Summer -0.670 0.000 -0.651 0.000 -0.632 was no significant correlation in spring. Winter -0,601 0.000 -0.661 0.000 -0.354 During summer, maximum length was neg- atively correlated with temperature. Length distribution by depth revealed that individuals larger than 155 mm, which is the length at first maturity (Reiiones et al. 1995), were generally found in stratum I1during summer and winter (Fig 4). The same was found for individuals ldrger than 165 mm in spring.

The calculated VBGC is shown in Fig. 5. 0 yr old fish were found in shallow water, while 3+ yr old fish were o ...... found in deep water. 1 and 2 yr old fish selected inter- mediate bottom depths (Fig. 6). A randomization test 350 based on the absolute maximum vertical distance between CDFs indicated significant shallow depth 280 selection for the age 0 group during summer and winter and significant selection for greater depths for the

TZIO age 3+ group during spring and summer (Table 5). Fish 5 CL $14~ 400 I I l I l

L, L, = 354 mm 70 k = 0.225 to = -1.194 300 - R' = 0.973 - 0 n = 1526

-E E 200 - - 280 +1 -E 210 fa - 0"140

70 0 I I I I I 0 1 2 3 4 5 6 0 nw,nw,w,=C-r------2ZzR~~~&L g~~qw.w.nw.w.nw.n~*w

OIZ 06 1 OL I 08 1 OS I OS 1 OE l

02 I 01 I

06 06 OL 09 OS 0 f OE

PLI 2 PZ P'f z L I 9ZZ 8lZ 9'91 IZ

Z91 z-02 b.6 1 8'5 1 981 8'LI P S1 L l Z'9 1 S I P'S l

9'bl 9'P I 254 Mar Ecol Prog Ser 166: 247-257, 1998

Table 5. Indexes of bottom depth and temperature selection older than 3 yr did not generally occur in shallow water by season. S: index of bottom depth or temperature selection; (stratum I),which was dominated by young fish. D: test statistic; p: probability of statistical significance of The temperature selection of red mullet showed a depth or temperature selection based on the randomization test described in the text pattern consistent with the depth distribution pattern. Younger fish selected high temperatures within the ranges we studied here (Fig. 6). A randomization test Age 0 Age 1 Age 2 Age 3+ 1 I indicated significant selection of the age 0 group for Depth high temperatures during summer and winter and of Spring the age 1 group, likewise, during winter (Table 5).A S -455.13 -71.27 545.71 935.90 randomization test did not indicate any significant D 38.43 25.36 37.47 67.21 temperature selection in spring due to the small tem- P 0.593 0.759 0.281 0.005 perature variation with depth. Summer S -766.71 -439.78 47.10 640.06 D 67.67 34.43 31.42 52.91 P 0.015 0.291 0.714 0.004 Maturity Winter S -668.41 -631.46 -66 55 482.11 All individuals collected during summer and winter D 62.88 56.95 13.09 26.82 were immature or resting (Stage I). During spring (late P 0.049 0.089 0.962 0.764 March to early April) most individuals were in eariy or Temperature late maturation (Stages 11, III), but no spent individuals Spring (Stage IV) were found, indicating that spawning takes S 495.40 398.39 -68.01 -215.14 place later in the season. Percentages of the different D 42.78 37.95 19.58 40.44 P 0.624 0.791 0.989 0.984 maturity stages by age group in shallow waters (stra- Summer tum I) and in deeper waters (strata I1 and 111) are shown S 3049.68 1010.42 387.70 1018.23 in Fig. 7. Only a very small percentage of O+fish were D 76.77 45.56 34.13 43.90 in advanced maturation in stratum I, whereas most P 0.002 0.741 0.922 0.748 individuals were in an advanced maturity stage in Winter strata I1 and 111. This is also indicated by the GSI S 654.83 607.65 457.93 287.18 (Fig. 8). It seems, overall, that the movement to mid- D 53.25 51.93 39.35 44.047 P 0.005 0.003 0.150 0.198 shelf depths is associated with reproduction.

Male Male

Stratum I Strata 11,111

0 + 1 + Female Female Stratum I Strata II,III 100% 80%

60%

40% Fig. 7 Mullus surmuletus. Percentage of the maturity 20% stage (see Table 1) by stratum, sex and age in 0% spring; 0+: 0-0.99 yr olds; 0 + 1 + 2 + 3 + 0 + l + 2 + 3 + l+. 1-1.99 yr olds; 2+: Ages Ages 2-2.99 yr olds; and 3+: Stage 1 Stage 11B Stage Ill Stage 1 • Stage 11H Stage Ill fish older than 3 yr Machias et al - Bathymetric distnbution and movements of red mullet 255

Male 4 Strata 11,111 -

2 2.5 Stratum I - 2 t

Female Strata 11, 111 - Stratum I 4

I -- -- 5 4 -- V)- 3.- Fig. 8. mullu us surmuletus. Mean 2 -- gonosomahc index (GSI), with t 95 % confidence intervals, by stra- 1 -- tum, sex and age, in spnng; age 0 ;l;/; I classes as in Fig. 7 0 + 1 + 2 + 3 + 0 + 1 + 2 + 3 + Ages

DISCUSSION deeper waters, the adults could benefit from a reduced basal rnetaboiic rste snr' icc:casc:! Kfe expectaiicy at There is a well-known general trend for size to lower temperature (Macpherson & Duarte 1991); there increase with depth in most demersal fishes. The gen- is also a density-dependent variation in bathymetric eral nature of this size-depth relationship could derive pattern (Swain 1993). Since, in the Mediterranean, the from migratory or diffusive movements to deeper water temperature remains virtually constant below a water during ontogeny (Cushing 1975). The prepon- depth of 160 to 200 m, Macpherson & Duarte (1991) derance of the positive size-depth relationships in fish suggested that ths behavior represents an inherited species may reflect a fundamental aspect of fish life evolutionary response and, therefore, has a genetic history or it could be the result of sampling artifact, e.g. basis. size-dependent catchability, or selective fishing pres- According to our results, Mullus surmuletus is sure (Stefanescu et al. 1992). However, sampling arti- among the species that show a positive relationship facts are unlikely to account for this in our research between mean length and depth. The analysis re- survey data, because the sampling gear and methods vealed that the smaller individuals tend to be found in were the same throughout the sampling, a reasoning shallower water during all seasons, whereas the larger previously followed by Snelgrove & Haedrich (1985). fish tend to be found in deeper waters during summer Additionally, selective fishing pressure is also unlikely and winter. In other words, smaller individuals tend to to be the reason for this observation, since it is inten- occupy warmer (high energy cost) shallower (high sive throughout the sampling area (Tsimenides et al. resource) grounds, and the older individuals colder, 1991). A further question is whether the relationship deeper grounds. reflects a progressive increase in fish size with increas- A change in the distribution of Mullus surmuletus ing depth. occurs in spring just before spawning, which takes The phenomenon of older specimens being found at place during April-May (Refiones et al. 1995). This greater depths has sometimes been taken as a rule change is mainly attributable to a movement of indi- (Heinke's law), as in the case of plaice (Harden Jones vidual fish from stratum I to stratum 11. In addition, 1968). The occurrence of smaller, younger individuals individuals in the final stages of gonadal maturation in shallower water and the movement towards deeper were generally absent from stratum I but abundant in waters during ontogeny must involve a substantial strata I1 and 111. Thus, final maturation seems to be advantage. It has been suggested that, by migrating to accompanied by a movement of fish to deeper water. 256 Mar Ecol ProgSer 166: 247-257. 1998

In Crete, wind-driven currents flow in the coastal area shallow competitive stratum I for the minimum neces- in April-May (Tselepides et al. 1996). These currents sary time. may prevent larval dispersal away from the continen- The movement of red mullet follows a definite pat- tal shelf. Furthermore, mullids have a pre-juvenile tern and could be considered as migration. The fish are pelagic stage (Kendall et al. 1984, Watson 1996) which spawned in stratum 11, they are recruited in stratum 1 makes it advantageous to the mullids to settle in and subsequently return to the stratum in which they appropriate areas. were born to reproduce. The following points may be Stratum I is covered mainly by Posidonia and is the made: (1) there seems to be a bathymetric rather than. substrate where red mullet recruitment occurs (Gar- a geographical determination of the 'immature' and cia-Rubies & Macpherson 1995). The trend of increas- 'mature' segregation, so there is no 'homing'; (2) there ing mean length with depth in summer and winter is was a short-distance rather than a long-distance move- due primarily to recruitment during July-August and ment; and (3) we found no evidence for any seasonal secondarily to the occurrence of larger fish in deeper migration cycle. Nevertheless, this pattern seems to water. The change in the distribution during spring have an evolutionary origin, namely (1) a possible could be attributed to the concentration of the imma- adaptive value In the tendency to dissociate the mature ture specimens in shallow water and of fish approach- from immature individuals, and (2) a possible adaptive ing maturity in strata I1 and 111. The vast movement value of the timing of the 'evacuation' of the recruit- ii-oill Posidonh beds to deeper water seem-s tn be ment ground by older fish. related to the final stage of gonad maturation, rather The same pattern of 'inter-depth' migration, related than directly to fish size. Individuals l+ and 2+ yr old to reproduction, has been observed in deep-sea fishes captured in stratum I seem to be the fish that had not (Middleton & Musick 1986). A difference is that the red reached maturity or that happened to be on the edge of mullet, instead of returning to shallow water after stratum I. Alternatively, larger individuals may attain reproduction, continues dispersing into deeper waters. final maturation earlier than smaller ones and/or some The absence of a correlation between maximum length individuals may spawn in stratum I. However, results and depth in the spring could be due to a movement of the monthly surveys (Fig. 3) support better the first from deep water to stratum 11, bu.t there was no clear hypothesis. evidence for that. An ontogenetic movement (after maturation) to The stocks of red mullet in the Mediterranean Sea deeper water might have the advantage of reducing are considered to be heavily exploited (Caddy 1993). inter-specific competition (Warburton & Blaber 1992). The existence of bathymetric movements in this spe- Stratum I (Posidonia beds) is inhabited by a much cies has ecological but also management implications. greater number of species than the other strata (Tsi- From a management perspective, it has generally been menides et al.. 1991). It is the nursery ground of most assumed that closure of trawl fisheries during the sum- Mediterranean demersal species (Garcia-Rubies & mer months (June to September) and in waters shal- Macpherson 1995). However. inter-specific competi- lower than 50 m would be an effective means of tion in this stratum might be low as a result either of a increasing the biomass of fish available to the fisheries. temporary adjustment in the occupation of the shallow Results of this study suggest that this management pol- zone (Warburton & Blaber 1992, Garcia-Rubies & icy is successful in protecting the immature fraction of Macpherson 1995) or of a shift in feeding habits the population, but reproducing adults are fully vul- (Labropoulou & Plaitis 1995, Labropoulou & Elefthe- nerable to overfishing, especially during spawning. rlou 1997). The movement of larger fish to greater This might cause a decrease in recruitment by depth could be a part of a strategy for better exploita- decreasing the number of reproducing females. tion of the shallow zone by a greater number of spe- Much of what fisheries management tries to achieve cies. Although no cannibalism has been reported, fish is aimed towards maintaining a sufficient biomass of prey only made an important contribution to the diet of reproductively active fish to replenish stocks (Roberts red mullet larger than 171 mm (Labropoulou et al. 1997). As pointed out by Stergiou & Pollard (1994), the 1997). The latter are generally found in deep water multi-species, multi-gear nature of Greek fisheries (stratum 11). Therefore, size-selective habitat use may (and of the Mediterranean fisheries in general) poses also functlon as a defense against cannibalism on other difficulties in designing and implementing protective conspecifics. measures. They suggest that the creation of marine Intra-specific competition could also be relaxed by harvest refugia may be potentially well applicable in movement of larger fish to deeper water, e.g. by the case of the Aegean Sea demersal fishenes. Such means of a bathyrnetric adjustment triggered by first protected areas would allow a proportion of the stock maturity. Additionally, this timing results in the to grow to a relatively large size at which overall smaller, rapidly growing individuals inhabiting the fecundity is greatly increased. Machias et al.- Bathymetric distribution and movements of red mullet 257

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Edjtorial responsibility: Otto Kinne (Editor), Submitted: Aday 15, 1997; Accepted: March 2, 1998 OIdendorf/Luhe, Germany Proofs received from a U thor(s): May 13, 1998