Spawning Potential Ratio and Abalone, What is it.

Thought its time to share some correspondence that has taken place between abalone constituent reps and a fishery management PhD.

There are three types of assessment approaches that are being conflated here: a biomass based approach, The CDFW density-based approach and length-based spawning potential approach.
1. Biomass-based approach: This is the typical method we think of when we hear the term “stock assessment”. This approach takes estimates of densities, the sizes of animals, and biological information collected over many years. These data are put into a model that attempts to estimate the current and historical amount of biomass in the water across the entire range of the stock. As you could imagine, this requires lots of time, money, energy and analytical expertise. It also incorrectly assumes that our goal should be to manage towards some fraction of the historical level of biomass, even though we know that environmental conditions have changed over time and that might not even be realistic. It also reduces the chance of ever accounting for local variability in biology and fishing pressure. This type of model is typically used for large industrial scale fisheries. (our abalone fishery is not large scale although me might think so) - RAD input in ()

2. The CDFW density-based approach: this is the transect approach currently in use. To my knowledge, the state biologists count the number and size of abalone at 8 locations approximately every three years. If densities along these transects drop below a certain threshold then catches are reduced. This method incorrectly assumes that the transects are representative of the entire fishery. Besides some statistical corrections, no real stock assessment model is used. In other words, there is no relationship between density and spawning output.

3. length-based spawning potential approach. This method uses information on the sizes of animals in the catch and a measure of the length at which half the animals have reached reproductive maturity. These two pieces of information are easy and cheap to collect across many reefs and can therefore represent the entire range of the stock (at least much easier than doing transects across the range of the stock). Through some basic multiplication, the length of animals in the catch and the size at reproductive maturity can be used to calculate how much egg production the observed (landed catch as a representation of what exists in the water) animals possess relative to what an unfished size structure would produce. The key here is that it is not about counting actual numbers of eggs being released each year (which is what biomass-based assessments do). It is about the ratio between the sizes of animals in the catch relative to the sizes of animals we would expect to see in an unfished population (again, calculated using basic biological information and simple multiplication).
The beauty of this method is that there are internationally accepted standards for how much potential egg production an abalone stock should have, in order to sustain itself. This number is somewhere around 25-35% and is called the SPR reference point. This number was calculated with the understanding that abalones don’t always reproduce each year, and many of the larvae die in the water column or are eaten by predators. A good example of how SPR reference points change for species with varying reproductive capabilities is deepwater rockfish. It is known that these animals only spawn once in a while and that good recruitment classes are few and far between (much less than abalone). Therefore the SPR reference point for deepwater rockfish is closer to 0.6 (60%).
To clarify how the method works, imagine the following scenario. A new fishery develops in an untouched area. The sizes of animals in the stock are representative of an unfished stock. Therefore, the Spawning Potential Ratio is one. As fishermen remove animals from the water, they typically take the largest ones. If we were to measure the landed catch after several months of fishing we may see that the size of animals currently in the catch are smaller than on the first day of fishing. This might represent a spawning potential ratio of 0.8. As the fishery goes on, more and more of the bigger animals are removed and the ratio decreases. As a manager, we can use the size structure of the catch to determine when the spawning potential ratio drops below the threshold for which we have agreed to act, in our case somewhere around 0.25-0.35.

In response to your question about “Seems like, theoretically, we could be fishing legal sizes and then the next year size could drop dramatically for up to 10 years without a change in take or an indication that spawning has not occurred.  How do we make sure this doesn't happen? The answer lies in understanding your data, good spatial management and well informed abalone divers. If the size limit is set appropriately for a given area, and divers are taking semi regularly (ever few years) data on the sizes of emergent abalone (those that recently became mature and exposed) these types of poor recruitment episodes will be anticipated and managed for. Abalone are very localized - and therefore, by monitoring the size data in the catch and communicating trends in emergence (via a website or other means), it will be possible to make local recommendations to relax harvest at local scales until the reef is ready to harvest again.

~RAD