Document: AND-3-72-12

Density-dependent interactions among juvenile fish: Using partially-specified models to explore inter-cohort interactions .

PAUL, A.J.*, W.A.NELSON, E.MCCAULEY and J.R.POST

University of Calgary, Calgary, Alberta T2N 1N4 Canada ¹

Abstract:
Density-dependent interactions that occur among cohorts of juvenile fish can theoretically result in cycling populations and complex stock-recruitment relations. Using fully-specified population models, it has been shown that these dynamics may occur for bull trout (Salvelinus confluentus) populations which spend several years rearing in small-unproductive streams. However, in developing a fully-specified model assumptions were made as to the underlying mechanisms which defined juvenile interactions. This raises two concerns when fitting the model to empirical data. First, is model fit evidence that inter-cohort interactions exist or merely an artefact of the necessary but unsupported assumptions required during model specification? Secondly, direct estimates of model parameters using the data can suffer either bias or inflated variance if the incorrect functional relations were assumed. To address the problem of model mis-specification, we utilized partially-specified models in which unknown mechanistic relations were represented by unknown functions. We analyzed a 15 year time series on densities of juvenile bull trout using this partially-specified model. The model was used to both test for inter-cohort interactions and estimate density-dependent mortality rates. Results using the partially-specified approach indicate that variability in the bull trout time-series was driven by juvenile mortality rather than adult fecundity. Furthermore, juvenile mortality rates were dependent on total juvenile density and suggested that a linear function exist between the two variables. These conclusions support assumptions made in the fully-specified modeling approach; thereby, reinforcing the importance of inter-cohort interactions among juvenile fishes in populations with relatively long developmental time periods.

Keywords: partially-specified models; bull trout; density dependence; inter-cohort interactions

This abstract is being presented at: 4:30 PM in session:
Oral Session #62: Freshwater Fish Ecology.