Reef Check California – Why and how do we count juvenile rockfish?

Each month, Reef Check will answer a technical question regarding the monitoring protocol of our coral reef or rocky reef programs. If you have a question you would like answered, please email rcinfo@reefcheck.org.

Reef Check California – Why and how do we count juvenile rockfish?

California’s kelp forest ecosystems have a great diversity of rockfishes. These species have a life cycle in which tiny larvae are released from the females (in contrast to most fishes, they don’t lay eggs). These planktonic larvae spend several weeks to a month in the open water where they grow and develop into fully formed fish. The timing of the release of larvae, and the duration of their pelagic stage varies greatly among species. Once the tiny fish have grown to a few centimeters in length, they return to the kelp forest habitat. Often they arrive in great numbers and several species of rockfish, now referred to as recruits or young-of-the-year (YOY), arrive simultaneously in pulses throughout the summer season. These recruits are the fish that will grow up on the reefs where we see them as adult rockfish. Therefore, their arrival on the reefs is an important component of the kelp forest ecosystem and the variability in their numbers that arrive every year has great consequences for rockfish populations.

In addition to sizing and counting the adult rockfish, Reef Check California (RCCA) also counts these YOY rockfish as they arrive at our monitoring sites. Many of the species of rockfish are very difficult to tell apart when they have just recruited to the kelp forest because they often look very different than the adult individuals of the same species and many species look very similar as YOYs. Therefore, during our surveys, we combine all species of YOYs into one category on our datasheet. That way we can track the variability in recruitment of rockfishes to our survey sites but do not have to identify each of these small individuals to species level. Additionally, by counting YOY in their own category we also remove them from the small (<15cm) category of the respective species. Since their abundance is highly variable and seasonal, this removes a lot of variability from our rockfish counts in a category that also includes older juvenile individuals which have been on the reef for up to several years. Removing this variability increases the usefulness of this category because without the YOYs we can use this data to look at the juvenile rockfish populations at our study sites. In summary, RCCA’s protocol of counting YOYs in their own category allows us to track the important variability of rockfish recruitment to the kelp forest while at the same time we can track older rockfish populations without having to account for the variability of seasonal recruitment pulses.