Research Document - 2013/078
Assessment of lobster (Homarus americanus) off southwest Nova Scotia and in the Bay of Fundy (Lobster Fishing Areas 34-38)
By Tremblay, M.J., Pezzack, D.S., Gaudette, J., Denton, C., Cassista-Da Ros, M., and Allard, J.
Abstract
Lobster Fishing Area (LFA) 34 is located off the southwest coast of Nova Scotia, while LFAs 35-38 are in the Bay of Fundy. The status of LFAs 34-38 was assessed by a two-part framework assessment held in 2012 (July 10-12) and 2013 (February 12-14). This assessment includes data on the fishing seasons ending in spring 2012.
This document provides review and analysis for the assessment of LFAs 34-38. It reviews lobster population biology and ecology and stock structure. Based on the degree of spatial and temporal integrity of population characteristics, two assessment units are warranted: LFA 34 and LFAs 35-38. Data analyzed in detail in the current document are landings and effort geo-referenced to 10 by 10 minute grids, size frequency from samples of the catch, and size and catch rates from Fishermen and Scientists Research Society (FSRS) recruitment traps. In addition to the above, data from other sources on fishery independent surveys are accessed for discussion of ecosystem considerations and for reference points.
In LFA 34, landings for 2011-12 (23,292 t) were an all-time high and about one third higher than the last time LFA 34 was assessed (2006). Within LFA 34 there has been a spatial shift in landings since 1998-99, with an increased percentage of landings in the mid- and offshore (37% of the total in 2010-11 and 2011-12 versus 11-15% in 1998-99 and 1999-00). This is largely due to a spatial shift in effort. Total fishing effort in LFA 34 has either varied without trend or declined since 1998-99. The adjusted annual number of trap hauls ranged from 19.9-23.4 million; 2011-12 effort (21,181,579 trap hauls) was just below the mean for 1999-2012. The catch per unit effort (CPUE) of the whole LFA in the last two years (1.1 kg/trap haul) is 1.7 times that of 1998-99 and 1999-00. Changes in size structure in commercial traps since 1990 in LFA 34 were most evident at the larger sizes. The 95th percentile of female size became smaller in traps set in the nearshore Grid Groups and in two of the midshore Grid Groups. In the offshore Grid Groups there was no consistent trend.
In LFAs 35-38, landings in 2011-12 (8,467 t) were an all-time high, and are about double the landings the last time LFAs 35-38 was assessed (2007). Fishing effort has increased in LFAs 35-38, but is still lower than in LFA 34 on an area basis. In terms of total annual trap hauls, reported levels in 2011-12 (4,539,140 trap hauls) for LFAs 35-38 were 1.2 times those estimated for 2005-06. CPUE in the Bay of Fundy increased in all LFAs since 2005-06. In LFAs 35-38 as a whole, CPUE in 2011-12 (1.9 kg/trap haul) was 1.8 times that of 2005-06. The size structure in commercial traps in the Bay of Fundy showed some reductions in large sizes since the early 1990s.
Data from standardized traps designed to retain more sublegal lobsters, maintained by the FSRS, were used to evaluate abundance trends of sublegal lobsters in some portions of the stock assessment units. Most available data were for LFA 34. A standardized CPUE model and a Temperature Corrected Abundance Index (TCAI) for LFA 34 nearshore indicate that sublegal abundance in the last 2 years was higher than all previous years in the 13 year time series. The standardized CPUE of sublegals in LFA 35 is available only for 6 years, but the last 2 years were the highest in the time series.
Exploitation rates (ER) were estimated for the nearshore portion of LFA 34 for the period 1999-00 to 2011-12 using Continuous Change in Ratio (CCIR). A different method (Length composition analysis, or LCA), was used to estimate ER for all of LFA 34, and for nearshore, midshore and offshore portions for selected seasons. LCA estimates of ER for LFA 34 as a whole ranged from 0.71 to 0.77. CCIR estimates for the nearshore portion of LFA 34 ranged from 0.63-0.94 with an overall mean above 0.80. LCA estimates for the nearshore fell within this range, and indicate that ER is lower in the midshore and offshore (mean of 0.36 for 2006-07 and 2009-10). Evidence for an upward shift in ER in LFA 34 since 1999 is mixed. ER in LFA 34 have been high for many years based on the long-term consistency in size distribution in the nearshore. High ER in the nearshore portion of LFA 34 have not inhibited the substantial increases in lobster abundance in the last 10-12 years. Given that environmental conditions remain favorable for lobster, the current levels of fishing effort do not appear to threaten the sustainability of lobster stocks in LFA 34.
For LFAs 35-38, there are inadequate data for estimating ER. Partial results for a few years for a portion of LFA 35 suggest a lower ER in the upper Bay of Fundy than in the outer Bay and in LFA 34.
A precautionary approach proposed for lobster in LFA 34 and LFAs 35-38 utilizes reference points for the abundance of legal sizes based on both fishery dependent indicators (landings and commercial catch rate) and fishery independent indicators. The fishery independent indicator for LFA 34 comes from an industry groundfish survey that recorded lobsters in the catch; for LFAs 35-38 the number of lobsters per tow in the DFO Summer Research Vessel survey provides a fishery independent indicator. Both sources have shortcomings but provide a view of lobster abundance independent of the commercial trap fishery. All of the indicators for LFA 34 and LFAs 35-38 are above their proposed Upper Stock References (USR).
The biomass trends of potential predators of lobster indicate most are at low levels relative to the long-term mean and median. Given the current low biomass levels of most of these potential lobster predators, a near-term increase in the natural mortality of lobsters due to these species is not expected.
The estimated percentage of the area of the LFAs contacted by lobster traps was low (<0.1%). In the nearshore portion of LFA 34, the total area in contact is higher but still less than 0.2% of the total area. The fishery footprint was lower in the Bay of Fundy LFAs (0.02-0.03%). These estimates do not account for any movement of the traps either due to storms or while hauling.
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