DISTRIBUTION, ADVECTION, AND GROWTH OF LARVAE OF 



THE SOUTHERN TEMPERATE GADOID, MACRURONUS NOVAEZELANDIAE 



(TELEOSTEI: MERLUCCIIDAE), IN AUSTRALIAN COASTAL WATERS 



R. E. Thresher,' B. D. Bruce,^ d. M. Furlani,' and 



J. S. GUNN' 



ABSTRACT 



Ichthyoplankton surveys in southern Australian coastal waters indicate that larvae of the temperate 

 gadoid, Macruronus novaezelandiae, differed consistently in mean size and age between sample sites. 

 These observations are consistent with the hypothesis that larvae are being passively advected by 

 longshore currents from a spawning area on the west coast of Tasmania to habitats along the southeastern 

 and eastern coasts. The ages of larvae at specific points along the advection route vary, which suggests 

 there is considerable variation in rate of larval transport. Rates of larval growth increased exponentially 

 for at least the first 50 days of planktonic life, though the slope of the growth curve varies both between 

 years and between seasons. Growth rates also differ between sampling sites: early stage larvae (<15 d 

 postfirst-feeding) grew more rapidly at sites close to the spawning area, whereas older larvae {>25 d 

 postfirst-feeding) grew more rapidly the farther they were from the spawning area. Migration of M. 

 novaezelandiae to a specific spawning area and the subsequent transport of larvae away from this area 

 appears to be an adaptive response by the population to, on the one hand, regional differences in condi- 

 tions for larval growth and, on the other, changing needs of the larvae at different stages of their 

 development. 



Planktonic eggs and larvae of marine fishes are sub- 

 ject to dispersion (= diffusion) and advection ( = 

 transport or drift), topics of considerable theoretical 

 and empirical interest to larval fish ecologists (Smith 

 1973; Wiedemann 1973; Talbot 1977; Okubo 1980; 

 Naganuma 1982; Power 1986). The causes and con- 

 sequences of diffusion, aggregation and patchiness 

 of larvae are largely unknown due to problems of 

 sampling at an appropriate scale (Hewitt 1981). 

 Advection of larval fishes, however, has been fre- 

 quently documented and has been studied in some 

 detail (see Norcross and Shaw 1984). Temporal 

 variability in advection can have considerable im- 

 pact on rates of larval survival (Norcross and Shaw 

 1984) and has long been suggested to be a major 

 determinant of year-class strength in populations 

 subject to variable current regimes (Walford 1938; 

 Harden Jones 1968; Nelson et al. 1977; Bailey 1981; 

 Parrish et al. 1981). In at least some species, eggs 



■CSIRO Marine Laboratories, GPO Box 1538, Hobart. Tasmania 

 7001, Australia. 



=CSIRO Marine Laboratories, GPO Box 1538, Hobart, Tasmania 

 7001, Australia; present address: South Australian Department 

 of Fisheries, GPO Box 1625, Adelaide, South Australia 5001, 

 Australia. 



'CSIRO Marine Laboratories, GPO Box 1538, Hobart, Tasmania 

 7001, Australia; present address: Tasmania Department of Sea 

 Fisheries, Crayfish Point, Taroona, Tasmania 7006, Australia. 



and larvae are placed in currents that transport 

 them to larval and juvenile nursery areas (Parrish 

 et al. 1981). Even within species, however, the ex- 

 tent of adult migration and larval countermigration 

 varies widely between populations, presumably in 

 response to local hydrographic conditions (Gushing 

 1986). Eastern North Atlantic gadoid stocks, for ex- 

 ample, provide some of the classic examples of adult 

 migration to spawning grounds and subsequent 

 passive drift of larvae to nursery areas (Harden 

 Jones 1968); in contrast, larvae of at least some 

 Western Atlantic stocks of the same species develop 

 entirely in the immediate vicinity of the spawning 

 grounds (O'Boyle et al. 1984, Sherman et al. 1984; 

 Smith and Morse 1985). 



By comparison with their Northern Hemisphere 

 relatives, little is known about the reproduction and 

 larval ecology of southern temperate gadoids, 

 despite the fact that several constitute major fish- 

 eries. One species, the blue grenadier or hoki, 

 Macruronus novaezelandiae, supports such a fishery 

 in Australia and New Zealand, with combined an- 

 nual landings of approximately 100,000 t (tonnes). 

 Available data indicate that both the New Zealand 

 and Australian populations migrate each winter to 

 discrete spawning areas, located, respectively, on 

 the west coasts of the New Zealand South Island 



Manuscript accepted; August 1988. 

 FISHERY BULLETIN, U.S. 87:29-48. 



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