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Fishery Bulletin 97(3), 1999 



4.5 

 4 



-3.5 

 3 



+ 



so 

 o 

 J 2.5 



2 

 1.5 



Spring 



Summer 



Winter 



1 



1 



-r ""t ^ 



Temperature "C 



Figure 4 



Seasonal distribution of Pagrus pagrus abudance in 1 'C temperature intervals, on the 

 Cretan continental shelf. Log(A''+l ) = the average logarithm of number of individuals per 

 nmi-. 



strate is sandy and interspersed with patches of al- 

 gae and seagrass. At this depth range, bottom water 

 temperature is the highest during all seasons. Thus, 

 this depth preference is consistent with the tendency 

 for red porgy to select warm temperatures in rela- 

 tion to those available in other areas of its distribu- 

 tion (Manooch and Huntsman, 1977; Manooch and 

 Hassler, 1978; Pajuelo and Lorenzo, 1996). Although 

 red porgy were found to occur down to 250 m depth, 

 their densities were significantly greater at shallow 

 stations. Furthermore, specimens smaller than 186 

 mm predominated in the trawl catches, whereas rela- 

 tive abundances of larger specimens was low. High 

 densities of red porgy in shallow waters were due to 

 the large number of juveniles, i.e. specimens that 

 have not reached sexual maturity (Manooch and 

 Hassler. 1978). The lack of larger fish from the trawl 

 catches in shallower depths could not be due to gear 

 selectivity because larger specimens were occasion- 

 ally found in deeper waters, indicating that they 

 could have also been caught in shallower depths, if 

 present. A similar distribution pattern has also been 

 reported for the Indo-Pacific species Pagrus auratus 

 which has been studied in many different areas 

 ( Azeta et al., 1980; Kingett and Choat, 1981; Tanaka, 

 1985). 



Size (or age) tends to be positively correlated with 

 depth in many demersal fishes. Tremblay and 

 Sinclair (19851 reported that the mean depth of oc- 

 cuJTence tended to increase with age of cod in the 

 southern Gulf of St. Lawrence. Similarly, Sinclair 

 (1992) noted a positive correlation between age and 

 median depth of cod on the eastern Scotian Shelf. 

 Macpherson and Duarte( 1991) found that mean fish 

 length increased with depth in most south-east At- 



lantic and north-west Mediterranean demersal 

 fishes. Bathymetric trends in demersal fish distri- 

 bution may be linked to other physical factors corre- 

 lated with depth (e.g. temperature, salinity, bottom 

 type). Temperature is a key factor in the metabolism 

 of fishes and many studies have reported relation- 

 ships between temperature and fish distribution ( Fry, 

 1971; Nakken and Raknes, 1987; Rose and Leggett, 

 1989; Macpherson and Duarte, 1991). However it is 

 unlikely that fish distributions are determined by 

 physical factors alone. Biotic factors, in particular 

 prey abundance, have also been shown to be impor- 

 tant determinants of distribution. Productivity gen- 

 erally decreases with increasing depth and distance 

 from land; therefore prey resources are greater in 

 shallow waters. Haedrich and Rowe ( 1977) concluded 

 that selection for mobility and metabolic efficiency 

 should favor larger size in the relative barren deep 

 sea, whereas Macpherson and Duarte (1991) sug- 

 gested that the positive size-depth relationship in 

 demersal fish may result from age-specific differences 

 in temperature preferences. They argued that younger 

 fish occupy warmer waters, where food supply and 

 growth rates may be gi-eater, whereas older fish oc- 

 cupy colder waters, where they may benefit from 

 lower metabolic costs and greater longevity. 



The most striking aspect of our results, was the 

 lack of variation in the bathymetric pattern of red 

 porgy and the fact that depth distribution varied little 

 among size classes, during all seasons. The scarcity 

 of specimens larger than 187 mm in the bathymetric 

 range >50 m is noteworthy, especially for a species 

 with a life span more than 12 years (Vassilopoulou 

 and Papaconstantinou, 1992). Thus the absence of 

 mature specimens in trawl nets during the pre.sent 



