Reproductive biology of male franciscanas from Rio Grande do Sul, southern Brazil

Fishery Bulletin, Oct, 2004 by Daniel Danilewicz, Juan A. Claver, Alejo L. Perez Carrera, Eduardo R. Secchi, Nelson F. Fontoura

An analysis of the variation along the year of the values of relative and combined testes weight, and seminiferous tubule mean diameter, was employed to assess reproductive seasonality. Values of these parameters were compared between months when mating and conception occur ("reproductive months": November-March) and months when they not occur ("nonreproductive months": April-October). In order to increase the sample size of mature animals collected in reproductive months, data on testes weight from mature male franciscanas from Uruguay were included in the analysis (data supplied by Kasuya (1)).

The mean age at attainment of sexual maturity (ASM) was estimated through the DeMaster (1978) method and the logistic regression.

The DeMaster (1978) equation computes the mean as

ASM= [k.summation over (a=j)] a([f.sub.a] - [f.sub.a-1]),

where [f.sub.a] = the fraction of sexually mature animals in the sample with age a;

j = the age of the youngest sexually mature animal in the sample; and

k = the age of the oldest sexually immature animal in the sample.

The variance of the DeMaster method estimate is calculated as

var(ASM) = [k.summation over (a=j)] [([f.sub.a](1-[f.sub.a])/[N.sub.a] -1),

where [N.sub.a] = the total number of animals aged a.

The logistic regression approach fits a sigmoid curve representing the probability that a franciscana of age a is sexually mature to the distribution of sexually mature and immature animals by age as

Y= 1/(1/(1 [e.sup.a bx]) or ln (1/Y-1) = a bx,

where x = the age of the dolphin;

b = the slope of the regression; and

a = the intercept.

To obtain the age when 50% of the animals are sexually mature (Y=0.5), the last equation is simplified as ASM = -a/b.

Mean length and weight at sexual maturity was also estimated by the DeMaster (1978) method, by substituting age for length and weight, respectively. The method was slightly modified, as suggested by Ferrero and Walker (1993), and was calculated as

LSM = [Cmax.summation over (Cmin)] L([f.sub.1]-[f.sub.t-1]),

where Cmax = the length or weight class of the largest or heaviest sexually immature animal;

Cmin = the length or weight class of the smallest or lightest sexually mature animal;

L = the lower value of the length or weight class t; and

[f.sub.t] = fraction of mature animals in the length or weight class t.

The specimens were pooled into length and weight intervals of 4 cm and 4 kg, respectively.

The estimated variance of this method is also modified and is calculated as

var(MS)=[w.sub.2] [Cmax.summation over (Cmin)][(f.sub.i](1-[f.sub.i])/[N.sub.i]-1],

where [N.sub.i] = the number of specimens in the length or weight class t; and

w = the interval width, a constant equal to 4 in these cases.

For estimating age, length, and weight at sexual maturity, pubertal animals were grouped together with immature animals.

Results

The weight and length of the left testes ranged from 0.23 to 10.42 g ([bar.x]=2.60 g) and from 15.7 to 59.7 mm ([bar.x]=33.6 mm), respectively. The weight and length of the right testes ranged from 0.17 to 9.98 g ([bar.x]=2.62 g) and from 17.9 to 60.0 mm ([bar.x]=34.5 mm), respectively. The relationship of testes weight and testes length resulted in significant regression (P<0.0001) and correlation ([r.sup.2]=0.91; F=823.9; P<0.0001; y=[0.000012x.sup.3.33]). The male with the heaviest relative testes weight was 141.6 cm in length and 31.2 kg in weight, and its combined-testes weight was 20.1 g, which is 0.064% of its total weight. The mean of the relative testes weight from 23 mature males was 0.036% of their total weight.