A Bio-physical Model of Cod and Haddock Recruitment
Marine Scotland Science and external collaborators developed a fully spatially and temporally resolved bio-physical model of cod/haddock egg production, the transport, growth and survival of the pelagic early life stages, and their settlement to a demersal lifestyle.
It is widely acknowledged that the success or failure of recruitment to the fishery is critically dependent on early life stages, since there is generally a good correlation between the abundance of demersal juveniles and subsequent recruitment.
Figure - Schematic of the structure of the STEREO model
The egg production module of the model simulates the daily population egg production over the duration of the spawning season, at each location where spawning is taking place. Egg production is a function of parental characteristics (length, weight).
Daily egg production is coupled to a particle tracking scheme, where each particle represents a given number of eggs. Eggs are advected by the flow and the duration of the egg stage is temperature history dependent. After hatching, the larvae continue to be transported by the flow (the vertical distribution of eggs and larvae changes as they develop (ontogenetically following relationships derived from the literature).
Growth is estimated as a function of temperature history and mortality is size-dependent. Once the pelagic juveniles reach the minimum size for settlement, they explore the suitability of the seabed for settlement. Given the lack of detailed ecological knowledge of this transition period, a range of scenarios were explored:
- The carrying capacity of a given location with suitable characteristics (for example acceptable depth range) is limited, so any potential settlers in excess of the carrying capacity will remain pelagic (subject to increased mortality levels) until they find a suitable settlement location with sufficient spare capacity. Demersal growth and/or mortality can be prescribed to be density-dependent
- All juveniles are able to settle on a given location with suitable characteristics. However, demersal mortality is density-dependent (and growth can be prescribed to be, as well). Two different alternatives of the latter scenario have been tested. One allows free carrying capacity to be re-claimed as mortality reduces the number of settlers. The other one does not; once a certain amount of carrying capacity has been taken up by settling juveniles, it becomes unavailable to the population, even if the actual numbers of settlers decline due to mortality
The bio-physical model can replicate standard stock-recruitment relationships and allow us to manipulate its components (for example environment, characteristics of the spawning stock) to quantify their influence on fish recruitment.
We can also use the model to identify which components of the spawning stock (in time and space) make the greatest contribution to recruitment historically, on specific years or as a consequence of specific management scenarios.