BLAXTER: ONTOGENY, SYSTEMATICS, FISHERIES 



CONCENTRATION (no/X) 



25 



20 



20 4 



20 8 



21 2 



CONCENTRATION (no /i) 



50 75 



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216- 



Fig. 4. Vanation in concentration of microplankton in samples from 20 cm depth intervals in the chlorophyll maximum layer over the coastal 

 shelf of the Southern California Bight dunng March. 1976. Prorocentrum, tintinnids and copepod nauplii are all food items for larval anchovy 

 (from Owen. 1980). 



decades. Improvement in plankton nets and young fish trawls 

 means that vertical profiling and quantitative sampling have 

 finally come-of-age. This ability to sample quantitatively is the 

 single most important advance in allowing larval populations 

 to be assessed reliably and for allowing models to be tested. The 

 outcome is two-fold. The door is open for biomass estimates of 

 spawning stock from egg and larval surveys and for testing the 

 possible factors in the stock-recruitment relationship. Each of 

 these will be considered in the final part of this paper. 



/. Biomass estimation. — ¥or many years population dynami- 

 cists lacked good information on the absolute size of the spawn- 

 ing stock and regulation was largely achieved by minimum mesh 

 and landing sizes. Of late, as a result of catastrophic declines in 

 some species, whole fisheries have been closed or controlled by 

 quotas and total allowable catch (TAC). The use of TAC's has 

 been greatly aided by virtual population analysis and also by 

 sonar-based fish counting surveys; these give an estimate of total 

 stock size, the reliability of which depends on the extent of the 

 survey, the ability to identify the species in question and the 

 precision of the calibration of target strength. 



To supplement the results, estimates of spawning stock size 

 have been made on an ad hoc basis by counting eggs and larvae 

 and converting them into the parental spawning stock biomass 

 by a knowledge of fecundity, age distribution and sex ratio. Some 

 of the pioneering work was done by Sette and Ahlstrom (1948) 

 on Califomian pilchard and Simpson (1959) on North Sea plaice. 

 Saville, Baxter and McKay (1974) counted the demersal eggs of 

 the herring on the small spawning ground of Ballantrae Bank 

 in the Clyde. This was later extended by Saville and McKay 

 (see Saville, 1981) to herring larval surveys in the North Sea 

 and off the Scottish west coast. The biomass of Pacific hemng 

 is now routinely assessed from the intertidal egg deposition along 

 the coast of Canada and the USA as described in the recent 

 Nanaimo Herring Symposium (Hay, 1984; Haegele and 

 Schweigert, 1984). Similar, but ad hoc. data are available for 

 the northern anchovy from the work of Smith (1972), Parker 

 ( 1 980) and Picquelle and Hewitt (1983), for the Atlantic mack- 

 erel from Lockwood, Nichols and Dawson (1981) and Berrien, 

 Naplin and Pennington (1981) and for North Sea cod from Daan 



(1981). Some of these data give absolute measures, some relative 

 ones from year-to-year, often related to biomass estimates by 

 other means. 



This survey technique has notable disadvantages. It must be 

 done at a limited time of year and is obviously easiest to interpret 

 for one-off spawners. The survey must be done rapidly and as 

 near the spawning season as possible to overcome any errors 

 caused by mortality between spawning and sampling. Although 

 it can be applied to a closed fishery, the age structure of the 

 population is required to compute the aggregate fecundity, hence 

 scientific sampling of the adults is required. 



2. Stock-recruitment.— The relationship between the size of the 

 spawning stock in any year and the number of recruits it supplies 

 to the fishery subsequently is vital information for the regulation 

 of fisheries. This is specially true where recruitment overfishing 

 is prevalent as in the clupeoids. Over many years a stock-re- 

 cruitment relationship may be obtained empirically in any fish- 

 ery, but this is time-consuming and usually contains inexplicable 

 features. While, as might be expected, low spawning stock leads 

 to low recruitment, high spawning stocks may also give unex- 

 pectedly low recruitment, as the result of density-dependent 

 effects. Alternatively spawning stocks of a given size can yield 

 enormously different brood strengths, of the order of 10-100 

 times, in a quite unpredictable way. 



It is not surprising that the underlying causes of the control 

 of brood strength are of much interest to fisheries biologists and 

 have received the attention of experimentalists and modellers. 

 Most marine fish have a very high fecundity, of the orders of 

 tens of thousands to a few million. From such a starting point 

 mortality must be very high and it is surprising that brood 

 strength variations are not even more variable than is actually 

 the case. What then do we know of the mortality rate of eggs 

 and larvae in the sea? Are there critical periods when it is es- 

 pecially high? What are the causes of mortality? 



Hjort's original hypothesis, now some 70 years old, expressed 

 the view that a critical period existed after yolk resorption as 

 the larvae sought external food sources. This hypothesis was 

 supported by earlier rearing experiments in which very high 



