EXPATRIATION OF XYRICHTYS-NOVACULA (PISCES, LABRIDAE) LARVAE - EVIDENCE OF RAPID CROSS-SLOPE EXCHANGE

Larvae of Xyrichtys novacula (Pisces: Labridae) have been collected in the Middle Atlantic Bight (MAB) hundreds of kilometers north of reported adult ranges by both the Marine Monitoring and Assessment Program (MARMAP) and by our ichthyoplankton cruises. These larvae could be from a previously unknown population north of Cape Hatteras or they could have been transported north from southern populations. In order to examine these two hypotheses both biological and physical oceanographic data were considered, including size-specific larval distribution, larval age, vertical distribution of larvae, satellite imagery of sea surface temperature, and temperature and salinity measurements. Larvae captured during our 1988 ichthyoplankton cruises ranged from 3.11 mm to 13.13 mm in length with small larvae (less-than-or-equal-to 5 mm) found in association with the shelf break and larger larvae (> 5 mm) found distributed across the outer shelf and shelf break. Based on aging of daily otolith increments larval age was found to range from 7 to 36 days and backcalculated brithdates, derived from estimated age and date of capture, were found to be from mid-June to early August. Larvae occurred predominantly near the surface (less-than-or-equal-to 10 m) at night and deeper (> 10 m) during the day. Concurrent hydrographic data revealed that small larvae were found in shelf water (< 35 psu) but with underlying slope water (> 35 psu) at depths of 15-30 m suggesting that small larvae were in shelf water at night but either at the boundary with or within slope water during the day. Satellite imagery from 1988 revealed a warm-core ring offshore of our 1988 sampling area. An analysis of historical warm-core ring data from Northwest Atlantic Fisheries Organization data reports in conjunction with captures of X. novacula during the years 1984-1987 demonstrated that the presence of warm-core rings offshore was associated with the occurrence of X. novacula on the MAB shelf, north of Chesapeake Bay. The feasibility of northward transport associated with the Gulf Stream and cross-slope transport associated with a warm-core ring was examined with a simple model which demonstrated that the required transport velocities were within the range of velocities presented in the literature. It is suggested that larvae are rapidly transported from south of Cape Hatteras to the MAB shelf break by advection associated with the Gulf Stream and a concomitant cross-slope flow related with the western edge of warm-core rings.