Juvenile Rockfish Abundance Surveys using SCUBA
- Tom Laidig
Rockfish are an important component of the groundfish fishery in California. In recent years, the biomass of many these species of has declined. In an effort to learn more about this decline and to a possible rebound in fish numbers, personnel at the National Marine Fisheries Service (NMFS) have been studying these species, particularly the rockfishes (genus Sebastes). One area of study focuses on the abundance of the young fish as they come into nearshore habitats.
Rockfish are live bearers, releasing larvae that floats in the ocean currents for many months and transform into juveniles. During the late spring through early fall, the juveniles many species of rockfish settle out of the plankton to the kelp bed. Some of these fish remain in the kelp bed environment for years, while others leave within in a few weeks for deeper waters. By counting the abundance of these juveniles while they are relatively accessible in these shallow habitats, researchers can predict how many adults may survive to reproduce and complete the cycle. Thus, studying the juveniles allow us to gauge the population biomass years before the fish enter the fishery and help managers to better regulate the fish stocks.
In 2001, personnel from the habitat branch of the Santa Cruz Laboratory of NMFS began continuous annual monitoring for juvenile rockfish in the kelp beds off Pacific Grove. Two kelp bed sites were selected, one off of Coral St. and the other off the north side of Pt Piņos. These sites were chosen due to there relative open ocean conditions and there accessibility for shore diving. Scuba surveys are conducted throughout late spring and summer to count the number of juvenile rockfish of all species that settle to the kelp bed and nearshore environments. The surveys consist of two divers swimming in a direction and counting all rockfish. After a minute, the totals are recorded and then the divers select another direction and conduct another one-minute count. An attempt is made to complete at least 20 one-minute counts at each kelp bed. The kelp beds are monitored periodically throughout the field season, with a minimum of 120 one-minute counts per season. From this data, an annual index is produced for each species. This index is compared to indices from other areas and used to predict fish numbers in the future.
Summary to DateOver the past four year, 8 species of juvenile rockfish have been observed at these two sites. These are: blue, black, yellowtail, canary, olive, widow, bocaccio, and copper rockfish as well as the gopher /black and yellow complex. Gopher and black and yellow rockfish are hard to distinguish, and so they are counted as one species complex. In general blue rockfish are the most abundant species, followed by yellowtail rockfish. Olive and black rockfishes are the next most abundant, and the remaining species were only seen occasionally.
From the graph of the four most abundant species (Figure 1), blue and yellowtail rockfish were abundant in 2001-2003, but dropped in 2004. Yellowtail have been decreasing since their high in 2002. Olive rockfish were at low levels in all years, with the highest in 2001. Black rockfish have been increasing over the time span. With a predicted El Niņo in 2005, it will be interesting to see how the abundances of these juveniles will change.
- Dispersal & Recruitment
Study MethodsScuba surveys involve 2 divers on full scuba equipment with writing slates, a compass and a stopwatch. Also, some distance transects are being conducted, so a 30-m tape is also used.
Figures and Images
Figure 1. Average number of rockish surveyed per minute for 4 different species from 2001 to 2004.
Young-of-year canary rockfish (Sebastes pinniger). Photo: Ammann
Young-of-year blue rockfish (Sebastes mystinus). Photo: Ammann