Integrated Fisheries Management Plan

Lobster in the Southern Gulf of St. Lawrence

STOCK ASSESSMENT, SCIENCE AND TRADITIONAL KNOWLEDGE

 
 

Biological Synopsis

The North American lobster (Homarus americanus) is widely distributed in coastal waters from southern Labrador to New Jersey, USA, with the major fisheries concentrated in the Gulf of St. Lawrence and the Gulf of Maine. Lobsters are also found in deeper waters (to 750m) in the Gulf of Maine and along the outer edge of the continental shelf from Sable Island to off North Carolina.

Lobsters live in temperate waters requiring cold water in winter and sufficiently high summer temperatures to grow, produce and hatch their eggs. Juvenile and adult lobsters can tolerate waters from less than 0°C to approximately 25°C. Larval lobsters occur in surface waters between 6°C and 25°C, though a minimum temperature of approximately 10-12°C appears to be required for successful bottom settlement phase (stage IV). Larval development is temperature dependent and takes just 10 days at 22-24°C but over 2 months at 10°C.

The life history of the lobster can be divided into a planktonic and benthic phase. The planktonic phase follows the hatching of the eggs in July and August. The larvae go through the free-swimming period that lasts from 3 to 10 weeks depending on environmental conditions, mostly water temperature. The planktonic phase ends at stage IV when the larvae settle on the bottom.

Juvenile and adult lobsters can tolerate a wide range of salinities from 15 to 32 ppt (parts per thousand) but can be affected by low salinities associated with spring melts or heavy runoffs in shallow estuaries. Larval lobsters are sensitive to salinities below 20 ppt, and alter their depth by actively swimming to avoid low-salinity surface waters. Lobsters are crustacean and grow by periodic shedding of their carapace called molting. Molting lobsters are less resistant to low salinities than are hard-shelled lobsters due to the osmotic permeability of their carapace.

Lobsters are found on many different bottom types from mud and sand to cobble and boulders. Young lobsters require shelter to avoid predators so are more restricted in their habitat than larger lobsters. Newly settled and juvenile lobsters are most common in complex habitats such as cobble or gravel bottoms, As they grow they are increasingly mobile and can be observed in all types of habitats but in higher densities in complex rocky habitats.

Female lobsters reach maturity at different sizes over their geographic range, and this is thought to be controlled principally by water temperatures, maturing at smaller sizes in regions with high summer temperatures (Gulf of St. Lawrence, Southern New England) and at larger sizes in regions with low summer water temperatures (Bay of Fundy). The maturity indicator used for management purposes is the size at 50% maturity, which is the size at which half of the animals are capable of reproducing for the first time. Male maturity occurs at a smaller size under similar conditions. For successful mating the male needs to be similar in size or larger than the females. Males can mate with numerous females but in other lobster species it has been shown that there are consequences of having too few males resulting in small egg masses.

Female lobsters will reach the size at 50% maturity at 72 mm of carapace length in most areas of the southern Gulf and 75 mm in western Cape Breton and part of St. Georges Bay. Mating occurs between July and September. Generally, female lobsters extrude eggs one year after mating and carry the eggs, attached under the abdomen/tail, for nearly another year.

Ecosystem Interactions

Environmental conditions, such as water temperature, can influence the distribution of lobster as well as their catches. Over most of the southern Gulf of St. Lawrence the bottom temperatures are typically less than 3°C, which is not considered favorable thermal habitat for lobster. This constrains the distribution of lobster to the coastal water (i.e., < 30 m) where bottom temperature can reach over 20°C (e.g. central Northumberland Strait) during the summer. Overall, environmental conditions have been warming in the southern Gulf over the last decade. In particular, sea surface temperatures have been rising in all lobster fishing areas. The volume of the cold intermediate layer has decreased and its core temperature has increased since the late 1990s. This may favour an expansion of the lobster distribution. In terms of larval drift and survival, current observations and models suggest that the Northumberland Strait is essentially an isolated system (relying on itself for recruitment) unlike the rest of the southern Gulf.

Lobster is largely omnivorous and mainly feeds on decapods species (57%-84% of prey biomass), with rock crab being the single most important one (45%-78%). Lobster cannibalism has been observed but a substantial portion (39%-79%) of the lobster remains found in other lobster stomachs consisted of discarded carapaces from molting activities. The only demersal fish known to consume large amounts of live lobster is the shorthorn sculpin.

Stock Assessment

Introduction

Lobster assessments are conducted periodically and peer-reviewed through the Regional Assessment Process (RAP) coordinated by the Canadian Science Advisory Secretariat (CSAS). The process includes participation by industry stakeholders so that their knowledge about the fishery is taken into account. The target frequency for full assessments for each LFA is about every five years. The conclusions and management advice are available to the public through Stock Advisory Reports (SAR), Research Documents and meeting proceedings published on the web at the Canadian Science Advisory Secretariat (CSAS) website.

Stock Assessment

The stock status of lobster in the five LFAs located in the Gulf Region is assessed using indicators primarily based on a fishery-independent trawl survey in LFA 25 and part of LFA 26A and SCUBA surveys in LFAs 23, 25, 26A, and fishery-dependent data from DFO official catch statistics, at-sea sampling (annually in P.E.I), and voluntary recruitment-index logbooks (annually in P.E.I).

In the absence of direct estimates of population abundance or biomass, stock status of lobster is based on a number of indicators including landings, catch per unit of effort, fishery-independent trawl and SCUBA surveys, fishing effort, exploitation rates, size frequencies, sex ratios, levels of pre-recruits, and proportion of egg-bearing females in the catch.

Stock Scenarios

There is no predictive tool for the lobster fishery to describe stock prospects or population trends. However, a retrospective analysis indicates that lobsters in the southern Gulf as a whole continue to be in high abundance with landings above the long-term median except in the Northumberland Strait. The lobster fishery in the southern Gulf continues to have high exploitation rates and to be dependent on new recruits making it vulnerable to recruitment fluctuations. The increase in the percentage of empty traps during the fishery in several areas also corroborates the interpretation that the fishing pressure on the lobster stock is high.

Recent multiyear management plans aimed at increasing egg production seem to have had a positive effect on lobster production in the southern Gulf as a whole. The only area that systematically shows negative indicators is the central Northumberland Strait. One possible cause is that the timing of the opening of the fishing season in LFA 25 (mid-August) is detrimental to the reproductive potential of the stock (i.e., egg production).

Research

The broad objectives of research studies are to increase our knowledge on the lobster biology and the coastal habitat in order to support decision-making on conservation issues for lobster stocks.

Recent and current studies include:

  • Collectors to evaluate lobster settlement and the biodiversity of species settling in the coastal habitat
  • Effect of exposure to environmental contaminants on juvenile lobster
  • Lobster landing and effort monitoring project using an electronic data logger
  • Ecosystem processes in Northumberland Strait
  • Impact of scallop harvesting activities on coastal habitats and associated species
  • Study on lobster benthic stages and habitat mapping
  • Internal organ pathology associated to lobster shell disease
  • Variability in trap catches from at-sea sampling during a spring fishery
  • Protecting window-size female lobster to increase egg production
  • Lobster thermal habitat in the southern Gulf of St. Lawrence in relation to climate changes

Aboriginal Traditional Knowledge

Aboriginal Traditional Knowledge (ATK) is recognized in this fishery as a source of information. Where aboriginal organizations are able to share ATK, DFO will consider it within the context of the management frameworks.