Northern rock sole (Lepidopsetta polyxystra) represent a commercially and ecologically important groundfish in Alaska, and like other flatfish species, they undergo a metamorphosis during the transition from a pelagic to benthic lifestyle. To date, the North Pacific Research Board has funded a series of projects examining habitat processes on the distribution and vital rates of this species in nursery areas around Kodiak, Alaska. Collectively, these projects have identified several important habitat variables for growth and survival. However, the mechanisms by which larvae are delivered to juvenile nurseries rock sole remain poorly understood and have raised several fundamental questions regarding essential fish habitat for this species. For example, do larval rock sole select habitat at settlement? If so, what habitat variables contribute to settlement and early post-settlement survival? Finally, do these early settlement processes subsequently contribute to overall variation in abundance, growth, and survival in post-settled juveniles among nursery areas?
Figure 4. Image of the 2-m columns used in thermocline trials at the Newport Laboratory.
Recently collected field data from known northern rock sole nursery sites show that newly settled rock sole juveniles are initially restricted to the shallowest region of juvenile nursery areas (< 3 m in depth) where substrates are unfavorable for burial and infaunal prey items (Fig. 3). One hypothesis is that thermoclines in deeper regions of the nursery prevent access to favorable substrates during settlement. Indeed, temperature data from the Kodiak nursery sites show 2°-3°C of stratification in deeper parts of the nursery whereas shallow regions remain unstratified throughout the settlement period. In addition, a recent AFSC study of juvenile rock sole distributions along the Inner Front of the Bering Sea suggests that changing thermal regimes control rock sole larval distribution and supply to nursery sites. To investigate these processes further, rock sole larvae were reared for a series of behavioral experiments at the AFSC’s Newport Laboratory. After 90 days of development, individual larvae were introduced to a series of 2-m water columns nested in a large tank (2.5 m x 2.5 m x 2.5 m) where vertical position, behavior, and settlement was recorded over a 3-hour period (Fig. 4). Water column treatments ranged from isothermic (4° or 9°C throughout) to thermally stratified conditions with either 4°C temperature change over 20 cm (strong thermocline: ST) or 100 cm (weak thermocline (WT) across the water column. Results indicated that the larvae concentrate at the thermocline break but demonstrate more spatial variation in the water column when conditions are isothermic (Fig. 5). Rates of settlement were highest in the isothermal treatment at 4°C, suggesting thermoclines do restrict competent larvae from settling. These data help explain the narrow depth range occupied by newly settled rock sole in Kodiak nurseries and support observations of annual changes in the spatial distribution of juvenile rock sole in the Bering Sea. Ongoing research is examining the consequences of settlement in poor quality habitat and whether post-settlement dispersal limits occupation of more favorable areas of the nursery.
By Benjamin J. Laurel and MaryKate Swenarton (Rutgers University)
