The effect of timing of tagging on streamer-tag recapture rates for American lobster - Homarus americanus

Fishery Bulletin, July, 2003 by Michel Comeau, Manon Mallet

(1) [N.sub.adj]=Np,

with variance

(2) V([N.sub.adj]) = Np(1 - p).

The field study recapture rate (t) is defined as

(3) t = m[([N.sub.adj]).sup.-1],

where m = the number of tags returned.

The variance of t can be calculated by using conditional theory and an approximate variance of [([N.sub.adj]).sup.-1] (Seber, 1982). In this study, we used Monte-Carlo simulations to obtain the 90% confidence interval (CI) for t. The simulations were carried out in two steps to include variability associated with tag loss from the initial N animals tagged and released, and variability in recapture rate. First, assuming that the number of tags retained ([N.sub.adj]) follows a binomial distribution with parameters N and p, a random number ([N.sub.adj]) was selected from a Binomial(N,p), where N was set equal to the number of animal tagged during a particular study, and p equal to the proportion of tag retention estimated from the aquarium study. With this simulated [N.sub.adj] a conditional value of t was derived as

(4) [t.sub.c]=[m.sub.o][([N.sub.adj]).sup.-1],

where "~" = the simulated value; and

[m.sub.o] = the observed number of recaptures in a field study.

Secondly, a value for m was obtained from a binomial distribution with parameters [N.sub.adj] and a conditional recapture rate [t.sub.c]. A simulated recapture rate was calculated as

(5) t = m[([N.sub.adj]).sup.-1].

The process was repeated 1000 times and the 90% CI for t was defined as the 5% and 95% quantiles of the resulting 1000 Monte-Carlo values of t.

Results

Aquarium observations

Tag-induced mortality was observed within 30 min of tagging for two lobsters tagged in the late premolt stage (1.5%) and for four in postmolt stage (2.1%) but not for lobsters tagged in early premolt and intermolt stages. The autopsies revealed that all of these lobsters died from perforation of the pericardial sac and these lobsters were not used in our experiments.

Tag loss of premolt lobsters was associated solely with tag-induced mortality and this was restricted to 10% (13) of the late premolt lobsters (Table 1). Autopsies revealed that six lobsters died within three days of tagging from the rupture of the dorsal thoraco-abdominal membrane, one died after one day from the perforation of the pericardial sac, and six died between three to sixteen days after tagging from a perforated hepato-pancreas (Table 2). In contrast, only 4% (7) of postmolt lobsters died from tag-induced mortality (Table 1). Autopsies revealed that all deaths were associated with the rupture of the dorsal thoraco-abdominal membrane. Another death was not related to tagging (Table 2).

A total of 70% (134) of the lobsters tagged in the premolt stage molted between 30 June and 8 September (between 7 and 77 days after tagging) without any tag loss (Table 1). Tag-induced mortality represented 8% (15) of the tag loss (Table 1). Autopsies revealed that the majority (11) of the lobsters tagged in late premolt stage died from the rupture of the dorsal thorace-abdominal membrane (Table 2); the tag was still attached to the old membrane and had caused the rupture of the new one. The four remaining lobsters died from poor healing of the tissue at the point of entry and exit of the tag, revealing a punctured dorsal thoraco-abdominal membrane (Table 2). For all lobsters that healed poorly and later died, one side of the tag had been pulled inside the body cavity during molting. One death was not related to tagging but was caused by the crusher claw becoming wedged within the old carapace, thus preventing the animal from successfully completing the molting process. Furthermore, the autopsy for this lobster did not reveal any sign of tagging trauma.