ALTERNATIVE METHOD FOR THE CALCULATION OF MEAN TIME FOR THE ASSESSMENT OF SECONDARY PRODUCTION BY TRUE COHORT ANALYSIS

The development time for the stages of a given population is a key par ameter in the estimation of secondary production by the true cohort an alysis method and requires an accurate knowledge of it. For this calcu lation, it is necessary to determine the mean time. This paper suggest s and develops an alternative method for the evaluation of the mean ti me of the organism-time curve. To demonstrate the application and vali dity of this method, data on three generations of the marine calanoid copepod Calanus pacificus Brodsky were used in the Deep Tank of the Sc ripps Institution of Oceanography for 58 days (30 April-26 June 1981). Once calculated, the development time of the stages was assessed. A c omparison of the results with the existent method of Landry(Int. Rev. Ges. Hydrobiol, 63, 77-119, 1978) is carried out which mainly uses ite rations with respect to time for approximating the value of the mean t ime. A statistical comparison was carried out with the results obtaine d by both methods; Landry's procedure with iterations of 0.8, 1.0, 1.5 , 2.0, 4.0 and 12.0 h. Through a paired test, the differences in mean times and development times for the three generations obtained by both methods were statistically non-significant (alpha = 0.05) only for th e Landry's time increment of 1.5 h: P = 0.432, 1.000 and 0.266 for mea n time, and P = 0.820, 0.956 and 0.765 for development time, for gener ations 1,2 and 3, respectively. With Landry's procedure, it is observe d that while the time intervals are small or large from similar to 4.1 6% of the sampling interval used, the results differ from those obtain ed with the method proposed here, underestimating or overestimating no t only the mean time, but also all the parameters that depend on it. I t is considered that the mean time obtained by the 'quadratic' method developed here is more precise and simple than the iterative ation is based on only one interpolation, that it is really a median, is accura te.