Cephalopoda: Octopoda), in the South Aegean Sea (Eastern Mediterranean)

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Not to be cited without prior reference to the author

International Council for the CM 1998/M:44 Exploration of the Sea Cephalopods Committee

Seasonal and spatial changes in the abundance and distribution of Eledone moshata (Cephalopoda: Octopoda), in the South Aegean Sea (Eastern Mediterranean)

E. Lefkaditou, A. Siapatis & C. Papaconstantinou

National Centre for Marine Research, Agbios Kosmas, Heiliniko, 16604 Athens, Greece

ABSTRACT

Seasonal and spatial variations in the abundance, distribution and size composition of Eledone moschata , Lamarck (1799) are studied in the insular area of the South Aegean Sea. Samples were collected during four trawl surveys carried out between September 1995 and October 1996, over a total of 51 stations between 30 and 635 m of depth. Eledone moschata was caught up to150 m of depth around the Kyklades islands (western zone), where as, over the wider shelf region of Dodekanisos islands (eastern zone) up to 90 m of depth. Size frequency analysis indicated a series of microcohorts entering the exploited stock in sequence, during almost the whole year. At least three sub-populations have been shown to occur respectively in Dodekanisos, North and South Kyklades areas, by application of Kolmogorov-Smirnov test on length frequency distributions of the specimens, corresponding to different topographic and hydrological characteristics. The greater species abundance and recruitment intensity observed in Dodekanisos are probably related to the seasonal upwelling, generated by Etesian winds during summer in the eastern Aegean Sea. In the western zone, a series of individual spawning areas seem to occur, in the abrupt coastal waters around Kyklades islands, where slight diversification in periods of spawning and recruitment may result a greater number of sub-cohorts. Similar suggestions made for Todarodes pacificus distributed around Japanese islands, indicate that this is probably a common adaptive strategy of cephalopod populations in insular areas.

Keywords: Cephalopods, recruitment, Aegean Sea INTRODUCTION

Southern Aegean Sea is one of the most oligotrophic areas in the Mediterranean (Stergiou et al.,1997). Upwelling occur during the warm period of the year, specially along the coasts of the Eastern islands and the coast of Asia Minor under the influence of strong Etesian winds. Although not comparable to the great upwelling ecosystems occurring in the major oceans, the seasonal wind-driven upwelling and the standing gyres consist a significant source of nutrients in this area (FAO/GFCM, 1992; Christou et al.,1998). The populations of cephalopod species consist of one or at most two year classes, so that temporal fluctuations of productivity could have serious effects on their survival, spawning, recruitment and overall abundance. Several sub-annual cohorts in a year and pelagic larvae widely dispersed by currents, seem to be an evolutionary mechanism for many squid species, to face local recruitment failure (Caddy, 1983). Benthic cephalopods and specially those lacking a pelagic phase during early development, like Eledone moschata (Boletzky, 1975), to assure optimal conditions to spawn and grow, take advantage of upwelling events in coastal regions (Coelho, 1985; Costa & Fernandes, 1993). The musky octopus Eledone moschata (Lamarck, 1799) is a primarily Mediterranean species (Mangold, 1983), found particularly abundant in the Aegean Sea (Bertrand et al., 1998). The scope of the present investigation is to detect the probable relation of the variation in the abundance and distribution of Eledone moschata with the topographic and hydrological characteristics in the study area.

MATERIALS AND METHODS

South Aegean basin presents a quite complex morphology due to the high number of islands. Moreover, the different water masses entering the Aegean from the adjacent Levantine and Marmara Seas and the weather conditions affect the hydrology and circulation. Thus the study area can be divided in three sub-areas (Fignre 1) with distinctive characteristics: a) Dodekanisos islands located at the eastern zone and characterized by a wider continental shelf. This sub-area is typically under the domination of the high salinity (38.9*10'3

36 '"N 274SE

Figure 1. Map of the Southern Aegean Sea showing the position of the hauls in the study area and the considered sub-areas. The stations where Eledone moschata was caught are marked by full rectangles.

12 •Dodekanisos 0 North Kyklades 9 ~..c: ~ South Kyklades Cl ..>:: -~ w ::J 6 D.. U

o III IV Cruises

Figure 2. Seasonal variation of Eledone moschata CPUE (kg/h) by area September 1995 n =1876 15

o~~~~~+-~~~--*-~~ o 2 4 6 8 10 12 14 Mantle length (mm)

, December 1995 20 1\ / \ n=270 / \ I \ 15 / \, I 0~ I \ / " \ 10 / "\ / \ I 5 / I / / 0 ,---+= ~ 0 2 4 6 8 10 12 14 Mantle length (mm)

20 May 1996 n=692 North Kyklades 15

o 2 4 6 8 10 12 14 Mantle length (mm)

September 1996 20 n =2246 15

* 10

0 0 2 4 6 8 10 12 14 Mantle length (mm)

Figure 3. Seasonal length frequency distribution of Eledone moschata by area Four seasonal trawl surveys were carried out in 1995-1996, over a bottom area 2 of 24349 lan , in the southern Aegean Sea (Figure 1). Two depth strata: 0-150 m, > 150 m, were considered and the sampling was based on random-stratified design. The minimum and the maximum depths trawled were 30 m and 635 m respectively. Average CPUE was calculated only for the first depth stratum, where Eledone moschata was caught, separately for each one of the sub-areas. The mantle length (ML) of all specimens was measured to the nearest 0.1 cm. The length frequency distributions of the specimens in the three sub-areas were compared by application of the Kolmogorov-Smimov test. The mean values of surface temperatnre over each one of the sub-areas were derived by image analysis of monthly processed satellite thermal images from May to October of 1995 and 1996. The satellite information came from the A VHRR sensor aboard NOAA 11,12 and 14, supplied by the DLR (Deutsche forchungsanstalt fuer Luft und Raumfahrt e.V.)

RESULTS and DISCUSSION

Eledone moschata was the most abundant cephalopod, at least in terms of biomass, in the South Aegean Sea. It was caught up to150 m of depth around the Kyklades islands (western zone), whereas, over the wider shelf region of Dodekanisos islands (eastern zone) up to 90 m of depth. An average of 54 individuals per fishing hour was recorded for the whole period, attaining 118 individuals per fishing hour, in Dodekanisos islands, during autnrnn 1996. Recruitment accounts for most of this increase. Similar increase in the species CPUE, has been observed in Adriatic during late autumn when the new year class was supposed to be fully recruited (Manfrin Piccinetti & Rizzoli, 1983) In September 1995, CPUE values were higher in the North Kyklades, as compared to those in Dodekanisos and S. Kyklades (Figure 2). This was the result of the higher proportion of adults (ML>6 cm) in the catches at North Kyklades (Figure 3). The low representation of these sizes in the rest sub-areas, is probably due to earlier spawning and decrease of the adults due to post-spawning mortality. However, the size frequency of Eledone moschata caught along the Turkish coasts during September-October 1992 (Salman et al., 1997), peaked at 9 cm ofML. So it could be supposed that, at least in Dodekanisos area, matnre individuals have migrated for spawning towards shallower waters, where no sampling was carried out. Mandic & Sijepcevic (1983) also report that Eledone moschata migrates in very shallow waters during summer in the south-eastern Adriatic. The size composition in North and South Kyklades, especially that of May, indicates a series of microcohorts entering the exploited stock in sequence, during almost the whole year. On the other hand, CPUE values are lower in comparison to those in Dodekanisos. The wide diversity of identifiable cohorts in the population of Todarodes distributed around Japanese islands, were attributed to the existence of individual breeding areas, each one associated with a particular featnre of the continental slope. Spawning and hatching period may differ depending on the circumstances in each site and result diversification in growth rate and timing between sub-cohorts (Osako & Mudara, 1983). Similar suggestions might be reasonable for the population of Eledone moschata distributed in the narrow continental shelf around the Kyklades islands. This is probably a common adaptive strategy of cephalopod populations in complex ecosystems like insular areas. 27 1995 26 25 o o 24 e 23 .aro iii 22 0. ~ 21 I- 20 19 18

May June July Aug. Sep. Oct.

27 1996 26 () 25 o 24 ~ 23 .aro iii 22 0. . ~ 21 I- 20 19 18

May June July Aug. Sep. Oct. - Dodekanisos - North Kyklades '-- South Kyklades 4-. . Figure l Mean mothly sea surface temperatures over the three sub-areas of Southern Aegean Sea. Data is derived througll image analysis from the monthly • processed NOAA / AVHRR SST images covering the period from May to October of 1995 and 1996. Strong Etesian winds (NW -WNW) blowing over the SE Aegean during the warm season create temporary upwelling confinned by thennal satellite images (Theocharis et aI., 1990). Respective sea surface chlorophyll maps, do not show a remarkable increase of primary productivity during upwelling periods. However, high level of bioluminescence, due to development of diatoms, has been recorded in the area of bottom elevation near the slope by Narusevich and Tokarev (1989). Thus productivity variation generated by seasonal upwelling in combination with the wider continental shelf, may result the more intensive and of shorter duration recruitment, in Dodekanisos sub-area. A comparison of length frequencies between the two years' autumns shows their peaking in relatively greater size in 1996. The mean surface temperatures over the three sub-areas (Figure 4) demonstrate the effects of upwelling in the study area, showing that upwelling occuned somewhat earlier in 1996. This is the most likely explanation of the earlier recruitment at Dodekanisos and South Kykiades, in this year. In conclusion, it appears from the results of this preliminary investigation that recruitment and seasonal abundance of Eledone moschata are most probably related to the hydrology and topography of the study area. Upwelling events seem to play an important role in recruitment timing in the SE Aegean. However a more detailed and synchronous collection of oceanographic and biological data is needed to establish a certain relationship between environmental parameters and musky octopus catches.

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