ONTOGENY AND SYSTEM ATICS OF FISHES -AHLSTROM SYMPOSIUM 



•20 



.16- 



(J 



•12 



•08- 



HERRING 



WILD 



12 16 



LENGTH (mm) 



"20 



Fig. 1 . Comparison between range of condition factors (C.F.) as dry 

 weight/length^ of wild herring caught at sea by plankton net and reared 

 herring larvae near starvation (from Blaxter, 1976). 



Finally, the ageing of larvae by daily ring formation in the 

 otoliths should be mentioned. This technique was pioneered by 

 Brothers et al. (1976) on anchovy larvae and California grunion 

 following Pannella's suggestion that daily increments were being 

 laid down in the sagittae of some temperate fish species. The 

 findings were validated by rearing larvae in tanks and sampling 

 the population at intervals of 1-7 days. Struhsaker and Uchi- 

 yama (1976) supported these results from their work on the 

 Hawaiian nehu and subsequently the technique was widely 

 adopted in fisheries laboratories. Attempts by Geffen (1982) to 

 manipulate ring formation in cod, herring, plaice, salmon and 

 turbot larvae by varying the photoperiod, temperature and feed- 

 ing regimes did not lead to any consistent result — the ring de- 

 position was frequently not daily and the main determinant in 

 herring and turbot seemed to be growth rate— the higher the 

 growth rate, the higher the rate of ring deposition. Bailey (1982), 

 however, found otolith rings deposited daily over a 10-day pe- 

 riod in post yolk-sac Pacific hake larvae reared in tanks. Sea- 

 caught larvae with more than about 30 increments were less 

 satisfactory because of the appearance of different types of ring 

 and it was not certain whether they were daily. Dale (1984) in 

 a recent study of reared Atlantic cod otoliths using electromi- 

 croscopy, found daily rings in a 12L/12D cycle but not in the 

 dark. Daily ring deposition only continued, however, for a few 

 days post-hatching. 



Although the ageing of anchovy and grunion from daily rings 

 seems reliable, further validation experiments are required at 

 sea. This is conceptually difficult on a wild stock of larvae of 

 mixed age and it is notoriously difficult to remain over a single 

 population of larvae for many days. Mass release of reared 

 larvae into the sea remains an ambitious possibility. Perhaps 

 best of all such a release should be into some large enclosure 

 system initially free of a larval population. Validation experi- 

 ments must also test the more unusual environmental condi- 



tions which apply in high latitudes where, for example, daylight 

 prevails over the full 24 hours. 



Experimental Work 



The functional anatomy approach to taxonomy so elegantly 

 described in a recent review by Moser (1981) shows the extent 

 to which structure can be used to deduce function. The inter- 

 action of this approach with that of the experimentalist has 

 yielded much useful information. 



Since the 1950's increasing success in rearing marine fish 

 larvae may have provided the taxonomists with help as well as 

 some doubts as described in the last section. It has also led to 

 a wide literature on the physiology, behaviour and physiological 

 ecology of larvae (and the use of larvae in pollutant bioassay) 

 as biologists seized the opportunity to exploit such new and 

 valuable material. Perhaps the most credit should be given to 

 Shelboume (1964) for his extensive and painstaking rearing ex- 

 periments on plaice, and later sole, at Port Erin, Isle of Man. 

 These experiments undoubtedly led to the present wide practice 

 of marine finfish aquaculture with the expanding commercial 

 use of turbot, sole, bass, bream and gilthead. 



Rearing may still be considered as something of an art and 

 is often most successful in the hands of dedicated people with 

 a "feel" for what is right or wrong. Undoubtedly a breakthrough 

 was made in finding suitable food for larvae. It is significant 

 that both plaice and sole can take Anemia nauplii from first 

 feeding as can some races of herring. This resulted in another 

 U.K. focus for rearing at Aberdeen, and later Oban, developed 

 by Blaxter (1968) on the herring. Species with smaller larvae 

 (with smaller mouths) were only successfully reared when Las- 

 ker's group at La Jolla (Lasker et al., 1970; Theilacker and 

 McMaster, 1971; Hunter, 1976) developed the use of the rotifer 

 Brachionus plicatilis and the naked dinoflagellate Gymnodmium 

 splendens as small food items for early-stage larvae of species 

 like northern anchovy and jack mackerel. About the same time 

 Howell (1973) also used Brachionus to rear turbot larvae at Port 

 Erin. 



Subsequently a number of factors have been identified to add 

 to our corpus of knowledge on rearing. These include the need 

 for good water quality, with the interesting idea of "green water" 

 culture of larvae in fairly high densitiesofC/j/oreZ/a which seems 

 to damp out fluctuations in metabolites, and perhaps enhance 

 oxygenation as well as providing secondary feeding for the larvae 

 (e.g., Houde, 1977; Morita, 1984). Adequate light for visually- 

 feeding larvae and the need to prevent excessive bunching of 

 larvae or their prey are also important, as is the quality of the 

 food. Success or failure may now depend on the fatty-acid profile 

 of the Anemia nauplii which are still used by most workers in 

 the later stages of rearing. Artificial diets of encapsulated or 

 particulate food are also being developed but have yet to be 

 introduced as a standard technique for early rearing. 



Before turning to the extrapolation and application of exper- 

 imental data to modelling, mention must be made of Haydock's 

 (1971) and Leong's (1971) work on the induction of spawning 

 in the croaker and anchovy by pre-treatment with an appro- 

 priate photoperiod followed by hormone injection. This has 

 been applied subsequently to the menhaden by Hettler (1981), 

 and to many other species, and has become a standard method 

 for workers requiring eggs over long periods or at a specific time. 



We now have the widest knowledge of the development, be- 

 haviour and physiology of both anchovy and herring larvae (see 

 Fig. 2) but there are several species such as cod, jack mackerel, 

 mackerel, plaice and turbot which run them a close second. 



