Sometimes We Get Discards Data

And when we do, life is glorious. This week I have focused a lot of my time on more stock assessments but for the discards side of the assessments. Discards, as the name suggests, refer to any animals that are caught during fishing activities that the fishery discards or returns back to the ocean. Two reasons that fisheries have discards are that non-targeted fish get caught in the fishing gear (known as bycatch) and that fishing regulations prohibit certain catch (regarding size, age, and/or sex of the fish). An example of bycatch is that the Gulf of Mexico shrimp fishery, which is a major source of revenue for the coastal United States, sets out large shrimp trawl nets which often catch fish and sometimes turtles or marine mammals. Since the fishery permits include only the shrimps, the fishers have to return these non-target species back as discards. An example of fishing regulations prohibiting catch that I had to look up the other day is that the state of Mississippi prohibits the recreational catch of any large coastal or pelagic shark that is smaller than 37 inches, and the regulations include a maximum allowance of 1 large coastal or pelagic shark per person and up to 3 per vessel. If an angler catches outside of this regulation, they must return the shark back to the ocean.

Depending on the source of the discards (recreational or commercial fishery), the vessel crew or independent observers may keep quite detailed records of discards, and scientists use these records to inform management and to generate stock assessments. As I mentioned before, stock assessments are unfortunately not the most user-friendly way to retrieve data or information, as stock assessments are often multiple hundreds of pages of information, figures, and tables, and the figures and tables are not in a digital repository; the data is in the PDF version of the stock assessment report. If the data are available, though, these stock assessments will contain information on discards, which are quite useful when your ecological model includes fisheries. In the fisheries page of our model, we can add information about fisheries landings--the amount of biomass of fish caught by the fishery--and discards--the amount of biomass of fish discarded by the fishery. These data are helpful in simulating actual trends in fishing during the model runs and can help stabilize the model because these landings and discards are both sources of biomass removal from the model, which can help balance otherwise unchecked groups. Unchecked groups here may refer to any animals in the model without predators. You can imagine that without a predator, model groups may become so large that they crash the rest of the model by consuming too much biomass, but when we add in fishing efforts that may remove some of these model groups, the fishery acts as a pseudo-predator.

Since the discards--like the landings--likely change on a yearly basis, we can add the discards data to our model time series as another calibration tool. If we let the model run, we can compare the modeled discards to the discards trends from the stock assessments and evaluate how well the model reflects the real world. Therefore, these discards data act as a more-is-better performance tool for our research, where the more model groups we have discards data for and that we can use in our calibration, the more likely our model captures real world trends.

At this time, I cannot share any specifics about our work and our model regarding what discards we have and are using, how we performed calculations or created the time series, etc., since the work is ongoing, and sharing data at this stage would be highly irresponsible. I am hopeful that in the next few weeks I will have at least some pictures to share of this or associated work, so stay tuned!