Estimation of the capture efficiency of trawl gear. I: Development of a theoretical model

Abstract

Bottom trawl surveys are conducted almost worldwide usually with simplistic assumptions about the area swept clean of fish. There is also a common assumption, for lack of anything better, that the trawl is giving an unbiased sample both as to species and size of the local demersal fish abundance. The general feeling that the assumptions are wrong has not led to much quantitative improvement on them. This paper starts with a brief review of previous attempts to set up a general model of trawl herding and capture. From the stock assessment viewpoint it is the fish depletion rate caused by the whole system, trawler warps and trawl gear that is of concern. This is not as yet entirely measurable. The conventional meaning of efficiency for the gear research worker is output/input meaning, here, catch/encounters. The relationship between these two concepts is discussed. The paper goes on to show how work already published can be extended to estimate the absolute efficiency of the trawl net with bobbin groundrope. This can be done by species length group. The next step shows how to incorporate sweep efficiency and then otterboard effect so that efficiency of the whole gear back to the otterboards is derived. This also means that the effective spread in terms of encounters at the otterboards can be estimated. Part II of this paper applies the theory to previously published comparative fishing data.