Estimate Fish Length Research Articles

Estimating fish length and age at 50% maturity using a logistic type model

We propose a two-parameter logistic type model for estimating fish length and age at 50% maturity using the nonlinear least-squares method. The independent and dependent variables in the model are length (or age) and the corresponding arcsin-square-root transformed proportion of mature fish (Pi). The two parameters in the model are length (or age) at 50% maturity (L50 or A50) and instantaneous rate of maturation (K). A simulation study was conducted to examine the statistical behaviour of the proposed model in estimating L50 and K. The L50 was well estimated with a small bias (<1%) using the proposed model in all simulation cases. The K was well estimated (bias < 1%) when its true value and the variance in Pi were small. The proposed model was found to have relatively smaller bias than the probit and Lysack's methods in estimating L50 (or A50). To examine whether there are significant differences in maturation patterns between different groups of fish, we propose fitting the maturation data to the proposed model, and then conducting an analysis of residuals sum of squares to test whether there are significant differences in the fitted models.

Morphological Characteristics of Otoliths of Selected Fish in the Northwest Atlantic

The relationships between otolith dimensions and fish size for six demersal and two pelagic species were examined and found to be both curvilinear and species-specific. Otolith morphometric observations included length for all species examined and weight, width, volume, cross-sectional area and location of the focal point for selected species. Regression models relating each otolith morphometric parameter to fish length are provided. Statistical tests to examine correlation between left and right otolith dimensions were carried out and, for redfish, correlation in otolith morphology between sexes was also analyzed. There appeared to be no statistically significant difference between left and right otolith or between sexes. The significance of each parameter for estimating fish length was examined and a multiple regression model determined. Otolith weight appeared to have the best discrimination for estimating fish length as well as the easiest parameter to measure.

Assessment of errors associated with harbour seal (Phoca vitulina) faecal sampling

Six harbour seals, ages 4–8 years, were held as pairs in a 10 times 20 times 2 m tank filled with sea water, and on 60 occasions were fed a meal of a specific species of fish or cephalopod of known size. The tank was drained periodically, and harbour seal faeces were collected on a 0.5 mm sieve. Number and size of otoliths and beaks found in faeces were determined. Fifty‐eight percent of 670 fish and 37% of 36 cephalopods fed to harbour seals were represented by their otoliths or beaks in faeces. Estimated number of prey consumed was determined from the greatest number of left or right otoliths or upper or lower beaks collected in faeces. Estimated length ofprey was determined from measurements of otoliths and beaks recovered in the tank and relationships of otolith and beak measurements to prey length. Estimated number of fish eaten was not significantly different among pairs of harbour seals, but was different among species of fishes. Only 24–35% of fish species with small otoliths were represented in faeces, whereas more robust otoliths from other species were less apt to be completely dissolved. Estimated length of fishes was significantly less than lengths of fishes fed to harbour seals in 39 (76.5%) of 51 trials. Cephalopod beaks were not affected by passage through the harbour seal digestive tract. Amount of otolith dissolution was not related to species of fish; estimated fish length was underestimated by an average 27.5%. Although some (7.4%) of the otoliths were collected within 100 h after the fish were ingested, more than 90% were recovered within 24 h after the fish was eaten. Correction factors were developed which will allow researchers to estimate more reliably number and size of fish and cephalopod prey eaten by harbour seals.

The use of fish vertebrae in scats for estimating prey size of otters and mink

A method of using fish vertebrae in scats of otters and mink in order to estimate prey size is described. Significant positive correlations between fish fork length and length of abdominal and caudal vertebrae respectively were demonstrated and described for a series of reference fish of different species. Comparison of direct length measurements of fish fed to captive mink with length estimates obtained from vertebrae recovered in then scats and derived using the appropriate regression lines determined from the reference fish revealed a close measure of agreement and indicated that the method achieves a high level of accuracy. Use of vertebrae from scats for estimating fish length is considered to have several advantages over use of scales, and the method greatly extends the range, accuracy and value of the information that can be obtained from scat analysis.