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

Various causes can reduce the initial number of tagged animals, mainly tag shedding, tag-induced mortality, and death from natural causes (and Holt, 1957). In the present article, "tag shedding" refers to the physical detachment of the tag from a lobster and "tag-induced mortality" refers to the actual death of a lobster caused by the tagging process.

In the southwestern Gulf of St. Lawrence, lobsters are harvested either in the spring prior to the July-August molting season, or in late summer and early fall (early-August to early October, partially during and shortly after molting) (Comeau and Savoie, 2001). The purpose of our study was to estimate the level of streamer tag loss for the American lobster tagged before and after the summer (July-August) molting season by using aquarium observations. From the results of our aquarium study, we determined adjustments of the recapture rate in relation to the molt stage at the time of tagging.

Materials and methods

Aquarium observations

Two experiments were carried out at the"Aquarium et centre marin" (New Brunswick Department of Agriculture, Fisheries and Aquaculture) in Shippagan, New Brunswick, with lobsters captured in Bale des Chaleurs (47[degrees]52'N; 64[degrees]52'W). Because the main focus of our study was to investigate tag loss in relation to the molting stage, only males were considered because they have a higher probability of molting annually compared to sexually mature females (Comeau and Savoie, 2001). All lobsters caught were brought to the laboratory where carapace length (CL) and shell rigidity were recorded (the latter with a durometer) (Comeau and Savoie, 2001). In both experiments, lobsters were tagged by the same person to avoid variability in tagging procedure following the technique described by Moriyasu et al. (1995). Streamer tags manufactured by Hallprint Pty. Ltd. (15 Crozier Rd, Victor Harbor, South Australia, 5211 Australia) were used. As is routinely done in our tagging studies in the field (Comeau et al., 1998, 1999), tagged lobsters were kept in a holding tank for a minimum of 30 rain following tagging, and dead lobsters were removed from the experiment. Tagged lobsters were then transferred to large tanks partitioned with 25 x 25 cm individual compartments. These tanks were supplied with running seawater at ambient temperature. Lobsters were fed rainbow smelt (Osmerus mordax) twice a week. Lobsters were examined three times a day in July and August, and on a daily basis for the rest of the experiment. The date of tag shedding, of molting, or of death was recorded for each lobster.

Limited aquarium space prevented the use of a control group of untagged lobsters. However, autopsies were performed on all lobsters that died in the course of the experiments in order to identify the cause of death. The following tagging traumas, causing death, were identified (Krouse and Nutting, 1990): 1) perforation of vital organs, such as the pericardial sac and the hepato-pancreas; 2) rupture of the thoraco-abdominal membrane; and 3) necrosis or infection of lobster tissue at both the point of entry and exit of the tag.