“I think [weakfish are] coming back now. I am a diehard…and I am seeing evidence of 2 or 3 year classes in great south bay now where before…they’re not huge year classes, but they’re larger than what I’ve seen over the last 5 years or so, so I think we’re gonna see a few more coming back. It appears that the weakfish’s problems are environmental, not caused by fishing pressure, although obviously any fishing pressure that’s put on the fish doesn’t help them but weakfish…seem to be coming back a little bit, and the last stock assessment indicated that natural mortality is coming down.” – Charles Witek, DirksOutdoors podcast (Jan 2 2021)
Two basic parameters of MSY are BMSY, the biomass required to produce that maximum yield, and the maximum fishing mortality (FMSY) that would not overfish the stock. A more intuitive and related metric to the latter is the rate of exploitation, which can be thought of as the proportion of fish killed in a given year that are removed by fishing activities. Yet, the instantaneous rate of fishing (F) is an important estimate produced by a stock assessment and also functions as a measure of fishing effort. For commercial fishing purposes, determining the “most bang for your buck” means maxing out the relationship of catch to fishing effort, such that you understand how much effort nets you the most yield, and beyond which it would be diminishing returns. An example relationship is illustrated below.
Adapted from Gulland and Borema 1973
Beyond just catch, a more refined estimate with an eye toward the “sustainable” part of MSY is yield-per-recruit (Y/R). This method calculates a curve per cohort, or age class, to better understand what selectivity maximizes the yield while protecting the recruitment class. The graph below shows what the Y/R curve looks like when the fishery is selecting for ages 1-3. The marginal yield is the increase in total yield achieved by adding one extra unit of effort. The slope of the tangent of the curve is catch per unit effort (CPUE).
As is both evident from the graph and intuitive conceptually, the steepest slope occurs when you go from no fishing (F=0) to starting to fish (F > 0). Making an assumption that when marginal yield drops off to 10% of that initial maximum CPUE is where it becomes no longer worth it, stock assessment biologists can thereby estimate the corresponding fishing rate (F0.1) as a cutoff for how much effort to expend.
Different curves of Y/R reflected by different ages at first capture (Adapted from Sparre and Venema (1998))
I have recently started a new job in the Marine Bureau of NJ Dept of Environmental Protection! (Here is where I add the disclaimer that anything published on my blog is my own personal content, and do not necessarily reflect the views or positions of NJDEP.) I will still get to interface with Aves insofar as they interact with our fisheries, but given the new fish focus in my career, I am launching a new domain: birdv.fish
I will also be taking this opportunity to refocus/reorganize the content of my blog etc. there, though all of the content that is here will be archived and available there too. I will be looking into the best way to migrate email subscriptions if something needs to be done on that front, so stay tuned. Also, my upcoming retirement of this domain casts it back into the sea for a new owner. Originally, I snagged this domain name with hopes of turning this page more broadly into something of an avian ecology news blog, but I never got around to converting it from my own personal research/birding blog. Years ago, I took some time to put together more general interest content, but that has fallen by the wayside in my current endeavors. So, I leave it to the adoption of perhaps someone who can do this title more justice. I will still be here for now as I get the new site up and running, but please add your bookmark now (as you can see on the link, currently it is just a clone of this site!)
