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

Abstract--Streamer tags are commonly used to study the ecology and population biology of the American lobster (Homarus americanus). Aquarium observations suggest that streamer tag loss, either through tag-induced mortality or tag shedding, is related to the molt stage of the lobster at the time of tagging, and the molting event itself. Tag-induced mortality, where lobsters did not molt, occurred within eleven and sixteen days following tagging for lobsters tagged in postmolt (4%) and late premolt (10%) stages, respectively; whereas no lobsters tagged in early premolt or intermolt stages died. Tag-induced mortality at time of molting was observed for lobsters tagged in late premolt stage (11%), and tag shedding was observed for lobsters tagged both in early (25%) and late premolt (11%) stages, but was significantly higher (P=0.014) for lobsters tagged in early premolt stages. Autopsies revealed that lobsters died mainly of organ perforations (hepato-pancreas and pericardial sac) following the tagging process, and rupture of the dorsal thoraco-abdominal membrane during the molting process. The total tag loss was estimated at 4% for lobsters tagged after molting, and 27% and 31% for lobsters tagged in early and late premolt stages, respectively. There was no tag loss for lobsters tagged in the intermolt stage during four months of laboratory observations (July-October). To minimize streamer tag loss, lobsters should be tagged during the intermolt or postmolt stage. Based on field studies, recapture rates for lobsters tagged in premolt stage are always lower than those of lobsters tagged in postmolt stage. Furthermore, recapture rates during the second year, for lobsters that molt in the year following tagging, were drastically reduced, and no lobster was recaptured after four years at large. Finally, to account for tag loss during the first year at large, a minimal adjustment of 24.9% (SD 2.9%) and 4.4% (SD 1.6%) for the recapture rate of lobsters tagged immediately before and after the molting season, respectively, is recommended. Adjustments beyond one year at large are not recommended for the American lobster at this time.

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Tagging methods to study the movement, growth, and exploitation rate for the American lobster (Homarus americanus) have improved over the last 70 years. One major improvement in the mid 1960s was the introduction of an insertion tag called the "sphyrion tag" that is anchored to muscle tissue (Scarratt and Elson, 1965), instead of body tags (Templeman, 1935) or carapace-piercing tags (Wilder, 1953) used earlier. By the late 1980s, the sphyrion tag was replaced by another insertion tag called the polyethylene "streamer tag" (Landsburg, 1991; Moriyasu et al., 1995) initially developed for shrimps (Penaeus spp.; Marullo et al., 1976). Insertion tags have the advantage of being retained through a series of molts, thus providing information on long-term movement and more accurate data on growth.

Tag loss could greatly bias the estimate of population characteristics and fishery parameters (Ricker, 1975). For obtaining population estimates from mark-recapture data, tag loss generally refers to the reduction of the initial number of tagged animals by means other than fishing. In a series of tagging studies, Comeau et al. (1999) noticed a constant pattern of lower tag recovery rates for lobsters tagged in the premolt stage than for lobsters tagged in the postmolt stage. They indicated that the level of fishermen participation (recovery rate) could be a possible cause of tag loss because they noticed a steady decline of recapture rates where multiyear tagging studies were conducted. However, because the same type of streamer tag was used and each lobster was handled individually, the fishermen participation could not explain the difference between recapture rates for lobsters tagged in premolt and postmolt stages for a given year; hence possible tag loss at molting was suspected as the cause of the lower recapture rates (Comeau et al., 1999). Furthermore, in an attempt to estimate mortality rates, Comeau and Mallet (2001) used a mark-recapture model and simulations to evaluate the best estimator. They concluded that the level of tag loss is high and could be a serious problem for estimating fishery parameters for the American lobster if information on tag loss is not available.

Moriyasu et al. (1995) showed that sphyrion tag loss for lobsters held in aquaria varies between 3% and 23% depending on the molt stage at tagging. They also mentioned that lobsters tagged with sphyrion tags showed a significantly lower return rate (19%) than lobsters tagged with streamer tags (44%) in a recapture study in the field and suggested a possible lower level of tag loss among lobsters tagged with streamer tags. However, they did not estimate the tag loss for streamer-tagged lobsters. Recently, Rowe and Haedrich (2001) showed that the shedding rate for streamer tags in the field could reach 18% (40% for molted animals and 11% for nonmolting animals) after 8--12 months based on double tagging with a secondary carapace marking. They also found that streamer tag shedding was not related to sex or size, but they did not study the level of tag-induced mortality.