Status of Stocks & Multispecies Assessment Program
Advances in Skate Management and Research: New Population Model for Alaska Skates
Catch recommendations for species managed by the North Pacific Fishery Management Council (NPFMC) are made using a variety of methods depending on the quality of data available for a species or species complex. These methods are classified into hierarchical tiers, with Tier 1 indicating the highest quality data and Tier 6 the lowest.
Until 2008, all skates in Alaska were managed as Tier 5 species. Under Tier 5, a fishing mortality rate (F) equal to the natural mortality rate (M) is assumed to be the upper limit to exploitation, and the overfishing level (OFL) is calculated as M times the biomass of the population (as determined through research surveys).
While the Tier 5 approach generally provides a conservative means of setting catch limits, it ignores such important biological information as age at maturity and age and size composition. Population models can incorporate these and many other types of data and are the preferred means of stock assessment for commercially fished species.
In 2007, AFSC researchers Olav Ormseth and Beth Matta created an age-structured population model for the Alaska skate (B. parmifera) using Stock Synthesis 2, a computer modeling application created by NMFS scientist Rick Methot.
In an age-structured model, the population is described in terms of the proportion of the overall population at each age. Life-history characteristics (e.g., length, weight, maturity status) are all described as functions of age.
Inputs to the population are through recruitment, which describes the number of new skates being added through reproduction. Skates are removed from the modeled population through natural mortality and commercial fishing. Among other pieces of information, the model provides estimates of unfished biomass (the biomass that would exist in the absence of all fishing) as well as estimates of current total biomass and spawning biomass.
The creation of this model provides a good illustration of how the different divisions of the AFSC contribute to successful fish management. Research projects conducted by members of the RACE and REFM Divisions provided critical life history information; surveys conducted by RACE yield biomass and length composition data; skate age information is provided by REFM's Age and Growth Program; and fishery catch information comes from the Alaska Regional Office and from fishery observers.
The B. parmifera model was accepted by the Council in fall 2008 and was used to set catch levels using Tier 3. Under Tier 3, the F corresponding to OFL (the catch limit) is the F that would reduce the spawning biomass to 35% of its unfished level (F35% ).
The ABC (the catch target) is achieved by fishing at the F that reduces the spawning biomass to 40% of the unfished level (F40% ). Thus, the ABC is less than the OFL, and the difference between the two values provides a buffer in the face of uncertainty.
For 2009, the Tier 3 ABC for B. parmifera is 25,854 t. In contrast, the Tier 5 ABC for 2009 would be 30,487 t.
The difference between the two tiers is likely a result of the late sexual maturation of B. parmifera (12-13 years of age), which effectively reduces the spawning biomass of the population. Therefore, Tier 3 management provides for more conservative management of B. parmifera in the BSAI.
Target fisheries typically remove between 15,000 and 20,000 t of Alaska skates from the BSAI each year. The results of the modeling effort indicate that this level of incidental catch is sustainable. The model will be an important tool in the future when considering the possibility of directed fishing for skates.
