INTRODUCTION 



Ontogeny, Systematics and Fisheries 

 J. H. S. Blaxter 



IN the inter-war years work on fish eggs and larvae was Umited 

 to studies on horizontal and vertical distribution with a view 

 to completing our knowledge of the early life history of different 

 species. Resources for research were then much more limited 

 than they are today and most work was done on the important 

 food fishes. In the 1 950's a great expansion took place as fisheries 

 biologists realised how much a study of early life history would 

 be a key to solving some of their problems. This expansion took 

 place on a broad geographical and mtemational front, but great 

 credit must be given to the foresight and imagination of E. H. 

 Ahlstrom. who built up a team of biologists at La Jolla who 

 then and subsequently, played a major role m leading and de- 

 veloping this field with special reference to the fisheries of the 

 California Current. 



In the last two decades the output of publications has risen 

 at an exponential rate as evidenced, for example, by the 62 

 papers in the 1973 Early Life History Symposium held in Oban 

 (Blaxter, 1974) and the 139 papers in the 1979 Symposium at 

 Woods Hole (Lasker and Sherman. 1981). Furthermore, in a 

 selected hibhography of pelagic fish and larva surveys prepared 

 by Smith and Richardson (1979), some 1200 papers are listed, 

 most of them published in the last 30 years. Ahlstrom was 

 certainly a major catalyst in this reaction, but it is sad to record 

 that his obituary appeared in the Proceedings of the 1979 Sym- 

 posium, although he was still alive and present at the meeting 

 itself to impart his wisdom and expertise. 



It is proposed to discuss the post-war advances in our knowl- 

 edge of early life history stages under five headings: (1) as they 

 impinge on systematics and taxonomy. (2) the success and role 

 o{ experimental work in tanks and of modelling, (3) the scaling- 

 up of tank studies to large enclosures and embayments, (4) the 

 application oi sea surveys to test models, to investigate the stock- 

 recruitment relationship and to measure spawning stock bio- 

 mass, and (5) \he future. 



Systematics and Taxonomy 



A number of techniques have been developed to help in the 

 identification and classification of fish larvae. Since the devel- 

 opment of the skeleton and meristic characters are now so im- 

 portant in identification, techniques of clearing and staining or 

 x-radiography have become standard methods for examining 

 the internal osteology of larvae (Ahlstrom and Moser, 1981). 

 Morphometries and body pigmentation are also important and 

 are used extensively by Russell (1976) in his monograph on fish 

 eggs and larvae of the N.E. Atlantic. 



Rearing experiments have shown that the sequence of de- 

 velopmental events may also be specific in character. For ex- 

 ample the development of the acoustico-latcralis system and 

 swimbladder in herring as shown by Allen, Blaxter and Denton 

 (1976) is a long-drawn-out affair and quite different from that 

 of the larval anchovy as described by O'Connell ( 1 98 1 a) or the 

 menhaden or sprat. There are several larval features, such as 



the swimbladder and other internal organs, or features of the 

 labyrinth, which would help in the separation of similar-looking 

 species if only they were not obscured by fixation. 



Often the taxonomist (or fisheries biologist) resorts to count- 

 ing menstic characters such as vertebrae, fin rays, scales or gill 

 rakers. Yet many of these characters have been shown by ex- 

 periment to be labile and to respond to environmental condi- 

 tions during early development. The earlier work, mainly on 

 freshwater species such as the sea trout, was summarised by 

 Taning (1952). Since then a range of further studies by Fahy, 

 Lindsey (e.g., see Fahy, 1982) and others have confirmed the 

 earlier experiments, showing that temperature, salinity and oxy- 

 gen level influence meristic counts and that there is a critical 

 period when this influence operates. Little work has been done 

 on marine species although Hempel and Blaxter (1961) showed 

 that temperature and salinity both influence myotome and ver- 

 tebral counts in herring (the species in which stock separation 

 by meristic counts has been most widely applied). 



It seems likely that any environmental variable which influ- 

 ences the relationship between differentiation and growth will 

 affect the meristic count by determining the amount of embry- 

 onic tissue which is present when the differentiation into skeletal 

 units lakes place. The larval taxonomist needs to be cautious 

 in interpreting small differences in meristic values, especially 

 when they are related to clines or other types of geographical 

 distribution. That is not to say, however, that there is no un- 

 derlying genetic mechanism. The environment acts as a "fine- 

 tuning" mechanism. Whether this fine-tuning is accidental or 

 adaptive might well be worth discussion at the symposium. 



A warning also needs to be directed at morphometries. Rear- 

 ing experiments in different-sized tanks by Theilacker ( 1 980b) 

 show the influence of space on growth rates. Compansons of 

 reared and wild fish larvae, especially of herring by Blaxter 

 (1976), show that tank-reared fish are often shorter and fatter 

 than their wild counterparts at the same developmental stage. 

 There seems to be an interplay between diet and activity which 

 is enhanced by the confinements of the rearing tank. This makes 

 it difficult to extrapolate growth criteria from tanks, such as 

 condition factor, to establish, for example, the nutritional status 

 of larvae at sea (Fig. 1). 



A further and serious problem identified by the handling and 

 use of live larvae is the shrinkage caused by capture and fixation. 

 A number of workers such as Blaxter (1971), Schnack and Ro- 

 senthal (1978), Theilacker (1980a) and Bailey (1982) have ad- 

 dressed this problem but the most significant findings are those 

 of Hay (1981) on Pacific herring. Feeding larvae from rearing 

 experiments were released into the mouth of a plankton net at 

 sea and then fixed by various techniques after capture. Shrinkage 

 in body length ranged from a mere 5% to a massive 43% de- 

 pending on the technique. Extensive voiding of gut contents also 

 occurred. The implications of these results in morphometric or 

 feeding studies will not be lost on the present audience. 



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