Kelp Bass Essential Fishery Information

If a female has recently spawned, postovulatory follicles will be present in the ovaries. When we know what proportion of females have postovulatory follicles, we can estimate spawning frequency. In a previous study conducted 25 years ago over a brief portion of the spawning season, a small sample size indicated that female kelp bass spawn approximately every two and a half days. In 2013, we examined the current spawning frequency exhibited by kelp bass over the course of the entire spawning season.

Results coming soon!

Fish spawning is triggered by environmental cues. In marine environments, common cues include lunar and/or tidal flux. These cues are important for species to successfully reproduce and may indicate the best times for survival of fertilized eggs and larvae. Kelp bass may experience other environmental cues such as increased day length or increased temperature that drive them to form spawning aggregations, but there may be additional, closely related environmental cues throughout the spawning season that trigger spawning pulses. Understanding which cues trigger spawning in kelp bass is important for understanding how and why reproductive potential sometimes varies from year to year.

Reproductive hormones fluctuate with respect to environmental cues and may peak during spawning pulses. With support from the Reproductive Biology Lab run by Dr. Kelly Young at California State University Long Beach, we used enzyme immunoassays to measure the concentration of estradiol in the blood plasma collected from kelp bass over the course of the 2013 spawning season to estimate spawning periodicity.

We also looked at how hormone concentrations relate to fish size, gonad size and the presence or absence of post-ovulatory follicles produced by females following spawning events.

Results coming soon!

Kelp bass are serial spawners, meaning they may spawn many times over the course of a spawning season. As in other serial spawners, kelp bass ovaries contain eggs at several different stages of development; however, only the hydrated eggs will be spawned.

Batch fecundity refers to the number of eggs released in one spawning event. By determining the batch fecundity for several individuals over a wide size range, we can develop a batch fecundity-size relationship which allows us to estimate the batch fecundity of females measured in the field. Batch fecundity will be an important parameter for estimating reproductive potential of kelp bass.

Staff counted the number of hydrated eggs in the ovaries to determine batch fecundity for individual fish.

Results coming soon!

Example of oocyte developmental stages in bass:

A – Hydrated
B, C & D – Vitellogenic
E – Cortical alveolar
F – Primary growth

The age of many fish can be determined by analysis of their otoliths. Otoliths are hard structures located in the inner ear that grow as the fish grows by adding layers of calcium carbonate. The addition of layers is affected by seasonal changes in growth rate, so that calcium carbonate rings may form annually and can be counted similar to tree rings. By counting the rings on the otoliths we can estimate the age structure of kelp bass. In 2013 we collected 806 otoliths from kelp bass in the southern California bight and the data is currently being processed for publication.

While examining otoliths is a common ageing technique in fish, no one has ever validated that the ring pattern is annual across size classes in kelp bass. To examine this, we will keep kelp bass of several size classes in captivity for at least one year. Shortly upon capture we will inject a chemical marker called oxytetracyline (OTC) into the musculature that will be naturally incorporated into the otoliths. After a year, we will remove the otoliths and confirm the periodicity of the ring pattern.

Results coming soon!

Incorporating ecosystem function into fisheries management is a growing priority for managers. Marine protected areas (MPAs) offer a unique opportunity to study ecosystem health in the absence of (or under reduced) fishing pressure. One measure of ecosystem function that can be applied to MPAs is food web structure and predator-prey relationships. Animal trophic level shows how high in the food web an organism is ranked based on the foods it consumes (Figure 1). Estimates of trophic level may be used as an indicator of fishing impact, with fish typically feeding at lower trophic levels in heavily fished areas due to reduced food availability and/or degraded habitats. Stable isotopes of carbon and nitrogen in animal tissues are effective chemical tracers of trophic level and diet. Although isotope values are usually collected from fish muscle tissue, fin clips can be used to get non-lethal estimates of trophic level if values in the two tissues are comparable. 

Figure 1. Diagram of the step-wise enrichment in nitrogen isotopes that occurs with consumption, moving up a marine food web

In summer 2016-17 staff collected and analyzed isotope values from fin clips of Kelp Bass to characterize their trophic level and diet at reefs inside and outside of four MPAs in the Channel Islands (Davis et al. 2019, Testing trophic indicators of fishery health in California's Marine Protected Areas for a generalist carnivore, Ecological Indicators). Kelp bass are a useful indicator species for localized measures of food web structure because they are common, heavily fished and have high site fidelity. We found that isotope values of nitrogen in the fin clips of Kelp Bass matched those in their muscle tissue, so Kelp Bass could be sampled non-lethally from Marine Protected Areas using catch and release methods. Then we investigated whether Kelp Bass trophic level was a useful indicator of MPA effectiveness. Although MPA sites supported a higher biomass of kelp bass than reference sites, there was no consistent pattern in trophic level inside versus outside of MPAs. Gut contents from Kelp Bass showed that exploitation of transient pelagic foods like salps and tuna crab, especially during warm-water events like El Nino, may reduce the effects of MPAs on prey availability for this species. 

Our findings suggest that MPAs will not have consistent impacts on food web dynamics for higher level generalist carnivores such as Kelp Bass, and that effects may vary substantially even within the same region. MPA's may have stronger impacts on carnivorous fishes that depend specifically on bottom dwelling foods, such as California Sheephead, and this remains an interesting question for future research.