High-frequency patterns in abundance of larval Pacific hake, Merluccius productus, and rockfish, Sebastes spp., at a single fixed station off central California

Standard CalCOFI (California Cooperative Oceanic Fisheries Investigations) ichthyoplankton bongo net sampling was conducted every 2 h over 11 days at a single fixed station off central California in February 1995. A conductivity, temperature, and depth cast was completed after each tow to record corresponding hydrographic information. Larval Pacific hake, shortbelly rockfish, and other rockfish were enumerated and length measurements were recorded. Time–depth contours of water density showed a clear boundary, defining the mixed layer along the σ = 25.0 isopycnal, that appeared to fluctuate vertically on a 12‐h cycle centered around 50 m depth. Temperature–salinity plots showed a distinctly different pattern in the mixed layer when compared with water below the pycnocline. Length–frequency distributions plotted by survey day indicated a relatively continuous input of small rockfish larvae into the survey area rather than a resident population growing progressively larger each day. Catches of rockfish larvae were much higher in nighttime tows and varied by survey day, but were unaffected by the depth of the 25.0 isopycnal. In contrast, catches of Pacific hake did not vary significantly on a diel basis, but were significantly variable by survey day and were influenced by the depth of the mixed layer. Catches of Pacific hake larvae were significantly higher during warm‐saline periods (spicy water), while shortbelly rockfish were significantly less abundant under these conditions. Spectral analysis showed a clear 12‐h cycle in the depth of the 25.0 isopycnal and a 24‐h cycle in the catches of rockfish larvae, while Pacific hake catches and spiciness of the mixed layer appeared to have lower frequency cycles in excess of 48 h.