MEPS 174:107-121 (1998)  -  doi:10.3354/meps174107

ABSTRACT: Larval transport was investigated in a New Zealand fiord by examining the reproductive cycles and larval distributions of the sea urchin Evechinus chloroticus Valenciennes, and the hydrography in Doubtful Sound over 2 sampling periods (SP). These were from 27 September 1993 to 24 March 1994 (SP1993/94) and 27 October 1994 to 11 May 1995 (SP1994/95). Reproductive cycles of E. chloroticus were monitored at 2-weekly or monthly intervals in 3 populations in the fiord, while the transport and development of larvae were examined by plankton sampling every 2 wk at 4 permanent stations inside the fiord, and 1 permanent station outside. To determine water exchange in Doubtful Sound, an S4 current meter was deployed at the entrance of the fiord at 15 m depth. The fiord has an estuarine circulation pattern with a thin low salinity surface layer flowing seaward, below which is a compensatory landward flow of seawater. A major, synchronous spawning of E. chloroticus was observed during SP1993/94 which produced a widespread, single cohort of larvae throughout the fiord. In SP1994/95, spawning was fractional and asynchronous and resulted in 2 cohorts of larvae during the summer. Larval densities were an order of magnitude higher during SP1993/94 (2743.0 larvae tow^-1) than in SP1994/95 (155.3 larvae tow^-1). E. chloroticus larval distribution patterns indicate that there was a high level of larval retention within the fiord, consistent with the estuarine circulation pattern. During SP1993/94, average larval densities 4 wk after spawning ranged from 65.3 to 461.0 larvae tow^-1 within the fiord, compared to an average of 2.0 larvae tow^-1 at the outer site. Similarly, during SP1994/95 larval densities were consistently higher within Doubtful Sound than at the outer site. Larvae completed development within 4 to 6 wk in Doubtful Sound. These results suggest that recruitment of E. chloroticus in Doubtful Sound is by larvae that have originated, been retained and completed development within the fiord. Reduced larval exchange provides a mechanism for observed genetic differentiation of this population.

KEY WORDS: Echinoid · Sea urchin · Larval transport · Closed population · Recruitment · Estuarine circulation · Fiord · Larval biology · Evechinus chloroticus