ONTOGENY AND SYSTEMATICS OF FISHES-AHLSTROM SYMPOSIUM 



PHYSICAL A CHEMICAL INFLUENCES 



CIRCULATION 

 DIFFdSlON 



80UNDARV EXCHANGE 

 DISCONTINUITY EVENTS (STORMSj 

 PRIMARY PRODUCTION & 



ORGANIC RECYCLING 

 POLLL TION OR TOXICITY 

 ABIOTIC FACTORS (TEMPERATURE 



SALINITY. OXYGEN) 



IMPORTANT I ARVAL FISH INTERACTIONS 



LARVAl 

 PREDATORS 



MOSTLY 

 UNIDENTIFIED 



RECRUITMENT 

 ASSESSMENTS 



USHERY MODELS 



ECOSYSTEM MODELS 



MANAGEMENT STRATEGY 



LARVAL MORTALITY DETERMINED DIRECTLY IN LINKAGES A A B 



o 

 o 



tl'RREMLY rWPHASlZED STL:nrFS 

 STtlDIES FOR EMPHASIS 



SI I DthS ME IMMFniAlE 

 I ESSEk IMPORT ASCE 



DIRECl HACnONAI IINKAGES 

 I ISKAGES OE I ESSER IMPORTANCE 



Fig. 3. A generalised scheme for the main interactions between larval fish and their biotic and abiotic environment, providing a basis for 

 modelling (from Laurence, 1981). 



day as when fed at the same food level continuously. Clearly, 

 expeiiments need to be devised to test the effect of spatial rather 

 than temporal food patchiness. 



The evidence is thus accumulating, but very slowly, that lai^al 

 survival may depend on the extent and stability of microscale 

 food patches or interfaces, at least in some areas. It may be that 

 the rather high food densities required in small-scale tank rear- 

 ing do indeed apply to conditions in the sea and that such 

 densities are only found in patches. 



SCALING-UP 



Two major areas may be identified where rearing work has 

 been extended into large-scale containers. The first of these are 

 the large onshore enclosures and embayments used by the pres- 

 ent generation of Norwegian biologists; the second are the deep- 

 water plastic bags used by Scottish workers in Loch Ewe on the 

 Scottish West Coast. The Norwegians have achieved remarkable 

 growth and survival rates for herring and cod larvae, as high as 

 30-70% survival from hatching to metamorphosis, in shallow 

 4,000-60,000 m^ enclosures (Oieslad and Moksness, 1981; 

 Kvenseth and Oiestad, 1984). The Loch Ewe bags, which are 

 deep cylinders, of about 300 m\ have been used for rearing 

 herring and cod, but with much less success than the Norwegians 

 (Gamble et al., 1981; Gamble and Houde, 1984). 



Possibly volume itself is important, or more likely the ratio 

 between volume and wall area. The interface between wall and 

 sea water is not a natural one for fish larvae, feeding may be 

 difficult al the interface, and food may aggregate there in an 

 inaccessible form. Morita (1984) reports that Pacific herring 

 larvae have recently been reared in 20 m' tanks with a 46% 

 survival from hatching to a mean length of about 7 cm in 1 1 2 

 days. This spectacular result may have been partly a feature of 

 a fairly large onshore tank but also the "green water" technique 



mentioned earlier. Hunter (1984) suggests that the high survival 

 in some large tank or enclosure experiments is achieved by the 

 elimination of predators. To the present author a combination 

 of optimal feeding conditions and low predation seems to be 

 the likely cause. 



The events have been described so far in a topsy-turvy way, 

 in that sea surveys have always been the most widely-adopted 

 approach to problems associated with the early life history of 

 fish. The experimental and enclosure studies are the icing on 

 the research cake, although both Norwegian and Japanese work- 

 ers are seriously considering the possibility of restocking de- 

 pleted inshore fisheries or topping-up poor year-classes of cod 

 and herring by releasing reared late-stage larvae or O-group 

 juveniles. 



Sea Surveys 



These are expensive in terms of ship-time and manpower. 

 Originally designed to advance our knowledge of spawning 

 grounds, larval drift, and horizontal and vertical distribution, 

 they are often now linked to more practical aims. Nevertheless, 

 superb time-series exist for areas like the California Current and 

 North Sea as a result of the patience and foresight of earlier 

 workers like Ahlstrom and later workers like Smith and Saville 

 (see review by Smith and Richardson, 1977). Sea surveys have 

 always been a rich ground for innovative science, in terms of 

 sampling techniques, interpretation and usage. Experimenters 

 and modellers have provided a great boost for this work, allow- 

 ing new interpretations to be made and new hypotheses to be 

 tested. 



No more mention will be made of the matrix-filling role of 

 sea surveys— namely the completion of details of life history, 

 which is still taking place and has been much aided by the vast 

 improvement in egg and larval identification in the past two 



