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 isa key parameter in the estimation of secondary production bythe true cohort analysis method and requires an accurate knowledgeof it. For this calculation, it is necessary to determine themean time. This paper suggests and develops an alternative methodfor the evaluation of the mean time of the organism-time curve.To demonstrate the application and validity of this method,data on three generations of the marine calanoid copepod Calanuspacificus Brodsky were used in the Deep Tank of the ScrippsInstitution of Oceanography for 58 days (30 April–26 June1981). Once calculated, the development time of the stages wasassessed. A comparison of the results with the existent methodof Landry (Int Rev. Ges. Hydrobiol, 63, 77–119, 1978)is carried out which mainly uses iterations with respect totime for approximating the value of the mean time. A statisticalcomparison was carried out with the results obtained by bothmethods; 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 differencesin mean times and development times for the three generationsobtained by both methods were statistically non-significant( = 0.05) only for the Landry's time increment of 1.5 h: P =0.432, 1.000 and 0.266 for mean time, and P = 0.820, 0.956 and0.765 for development time, for generations 1, 2 and 3, respectively.With Landry's procedure, it is observed that while the timeintervals are small or large from –4.16% of the samplinginterval used, the results differ from those obtained with themethod proposed here, underestimating or overestimating notonly the mean time, but also all the parameters that dependon it. It is considered that the mean time obtained by the 'quadratic'method developed here is more precise and simple than the iterativeone because its evaluation is based on only one interpolation,that it is really a median, is accurate.