Conch, Lobster and Sea Cucumber

Benthic species are those that live on the sea floor, and there are a few of significant importance to commercial fishermen in Port Honduras Marine Reserve (PHMR). Regular monitoring of commercially harvested species like Queen conch, Caribbean spiny lobster and ‘Donkey Dung’ sea cucumber is carried out under the Commercial Benthic Species monitoring program. Due to indications of possible overharvesting in recent years in PHMR, the ongoing monitoring aims to determine population health of commercial benthic species and inform sustainable catch quotas.

As part of this monitoring, TIDE’s Community Researcher team measures mean size, population structure and population density or abundance for these species both outside PHMR and inside the different management zones within it (Replenishment Zones, RZs, where no fishing is allowed, and General Use Zones, GUZs, where fishing is allowed during the open season of each commercial species respectively). Results are then compared between these “fisheries independent” underwater visual surveys and “fisheries dependent” landings data. Tracking changes in these population assessments in the different zones allows us to understand what effect management zonation has on species dynamics, enabling TIDE to apply adaptive management to support sustainable fishing.

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TIDE Community Researcher collecting data during an underwater visual survey

 

 

Lobster

 

Each year, TIDE monitors lobster populations in underwater visual surveys conducted at the beginning and end of the closed season in February and June, respectively. On timed dives at each of the designated survey sites, TIDE’s Community Researcher team, overseen by Marine Biologist Tanya Barona and Science Director James Foley, measures carapace length (the length of the lobster’s hard upper shell, not including the tail), gender and maturity for each lobster they encounter. Abundance is then estimated from the number of lobsters encountered per hour. These measurements help us to assess the size structure of lobster populations. This enables us to monitor stock maturity over time, extremely important for predicting future productivity of the fishery and thus setting sustainable future catch limits.

There are signs that lobster are beginning to recover from population declines in 2012-14. Overharvesting is likely to have been the cause of this significant drop in abundance that did not recover during the closed seasons of the past several years, but an increase in abundance was finally seen during the 2015 closed season within all zones (Fig. 1). TIDE will continue to monitor closely to determine if this trend continues.

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Fig. 1: Mean lobster abundance by zone (Replenishment Zones, General Use Zones and Outside of PHMR), 2009-2015. Pink bars represent closed seasons.

Mean carapace length has decreased since 2013, even within RZs, suggesting possible illegal harvesting from these areas or increased migration of mature lobsters into surrounding GUZs (Fig. 2). This decrease in carapace length suggests reduced maturity of the populations living within the RZs, which can impact the GUZ through reduced spillover due to fewer reproducing lobsters protected within the RZ. 

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Fig. 2: Mean lobster carapace length by zone (Replenishment Zones, General Use Zones and outside of PHMR), 2009-2015. Pink bars represent closed seasons. Dotted line represents minimum size limit.

Lobster Management Recommendations:

An increase in mean carapace length during the most recent closed season is a good sign; however, there is still concern that lobster shades located close to RZs may be luring mature lobsters away from protected habitats. Further research is needed to determine this. Also, expanding the RZs to increase the amount of preferred habitat under full protection may help promote spillover effect, a key management tool for replenishing lobster stocks to the GUZ. Stronger enforcement is needed to reduce illegal harvesting. TIDE continues to work with Managed Access fishers to encourage more accurate reporting of catch data so that the fishery can be managed more effectively.

Lobster Juvenile Recruitment Study:

To further aid in management of the lobster fishery, TIDE conducted a lobster juvenile recruitment study. This research was aimed at determining the most favorable habitats for juvenile lobster settlement or “recruitment,” aided by the use of Lobster Attraction Devices (LADs). Understanding which areas are best at promoting juvenile recruitment will inform adaptive management in PHMR by ensuring the right areas are being protected to promote juvenile recruitment and increase spillover. Results of the pilot study showed the method employed to be effective at determining areas of juvenile recruitment. This study now needs to be scaled up to more areas before definitive conclusions can be drawn.

 

Conch

 

TIDE’s research team also monitors conch populations at the start and end of the conch closed season, from 1st July to 30th September. Shell length and lip thickness are measured for each conch encountered along set transects at each site. The shell lengths and lip thicknesses are then divided into size cohorts, grouped by standard increments (e.g. for lip thickness 0-<5mm, 5-<10mm etc.) in order to establish population structure based on size frequency distribution. This can be used to determine average maturity of the population and its reproductive capacity, or fecundity. Population density is also calculated as the number of conch encountered per hectare of area surveyed.

Results indicate conch density has dropped significantly in recent years and the closed seasons of 2013 and 2014 appear not to have fulfilled their purpose of increasing density by protecting conch during their reproductive period (Fig. 3). This perhaps could be a result of not having enough mature conch to reproduce due to overharvesting in previous years or poor reproduction due to natural events in previous years, or both. Mean conch shell length has remained fairly consistent (Fig. 4); however, average lip thickness decreased in 2015 (Fig. 5). Both shell length and lip thickness may serve as indicators for sexual maturity in conch, so changes in either of these measurements are important as they have the potential to impact fecundity of the population.

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Fig. 3: Mean conch density (number of conch per hectare of area surveyed) by zone (Replenishment Zones, General Use Zone and outside of PHMR), 2009-2015. Pink bars represent closed seasons.

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Fig. 4: Mean conch shell length by zone (Replenishment Zones, General Use Zone and outside of PHMR), 2009-2015. Solid black line represents minimum shell length at harvest. Pink bars represent closed seasons.

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Fig. 5: Mean conch lip thickness by zone (Replenishment Zones, General Use Zone and outside of PHMR), 2009-2015. Solid black lines represent minimum lip thickness at maturity, for males and females, according to Stoner (2012). Pink bars represent closed seasons.

Conch Management Recommendations:

Large drops in conch density in recent years are problematic, especially since the closed seasons have not been successful in allowing the population to recover. This fishery should continue to be closely monitored with a strong focus on enforcement to prevent illegal harvesting of this pressured species. Research into relationships between maturity, shell length and shell lip thickness will help determine how harvest restrictions should be set to allow sufficient time for reproduction (Foley 2016). This will be critical in protecting the future of this fishery. TIDE is also promoting involvement of fishermen in conch maturity research to help improve stakeholder understanding of the importance of harvesting only mature conch, enabling juveniles to reach sexual maturity and reproduce. This will promote long term sustainability, and incentivize fishers to record catch data more accurately.

Conch Lip Thickness at Maturity Study:

In 2015, TIDE began research to determine whether conch shell length, the currently used measure for maturity by the Belize Fisheries Department, is the best proxy indicator for determining sexual maturity of conch. It has been suggested from other data in the Caribbean that shell lip thickness may be a better indicator, which would in turn have an effect on conch harvest restrictions. The current shell length restrictions may not be protecting juvenile conch and this research will help us determine which is the best way to measure maturity so that the fishery can be managed more sustainably. Read more about this project here

 

Sea cucumber

 

Sea cucumber monitoring is conducted at the start and end of the closed season (1st July to 31st December). Divers measure length and width of all sea cucumbers found within a set radius of the designated sites, being careful not to disturb them to avoid them retracting. Sea cucumbers are subsequently brought to the surface to be weighed and then returned to their original location without harm. Population density is estimated by calculating the number of sea cucumbers per hectare of area surveyed.

Sea cucumber density has dropped significantly since 2011, in both Replenishment and General Use Zones (Fig. 6). There are signs of a slight recovery in the RZs in the past year, although not statistically significant. No recovery was observed in the GUZ in 2015. This is likely due to overharvesting, and such low population densities may make recovery more difficult as the chances of sea cucumbers encountering each other for reproduction is greatly reduced.

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Fig. 6: Mean sea cucumber density by zone (Replenishment Zones and General Use Zone), 2011-2015. Pink bars represent closed seasons.

Sea Cucumber Management Recommendations:

It is recommended that the sea cucumber catch quota is reduced by at least 50%, effective immediately, and reviewed annually as TIDE continues to monitor population density and maturity. Stronger monitoring and enforcement of a designated landing site for sea cucumber would also help to reduce illegal harvesting and incorrectly reported catch data. Finally, it is suggested that new RZs be introduced that protect preferred sea cucumber habitat, since the current RZs were established before a commercial fishery existed for sea cucumber and thus do not protect areas most conducive to sea cucumber reproduction. These measures combined will help sea cucumber populations to recover, important not only because of its commercial significance (in 2014 sea cucumber was the most valuable species in terms of catch per unit of effort expended; Foley et al., 2015), but also because sea cucumbers play an important role in maintaining water quality by eating decaying matter on the sea floor. If sea cucumber populations continue to decline there is concern that a buildup of detritus on the sea floor could decrease water quality, threatening coral reef and seagrass habitats in PHMR and harming other commercially important species as well.

 

Future

 

TIDE continues to monitor lobster, conch and sea cucumber populations under its Commercial Benthic Species Monitoring Program, and annual update reports are published to provide the Belize Fisheries Department with the most recent data as well as recommendations for management. (Our most recent annual report can be downloaded at the end of this page.) Results of TIDE’s research have revealed the need for adjustments to the current size restrictions. Continued monitoring is recommended as well as a reduction of the size limit for lobster and integration of conch lip thickness into size limit legislation for conch to ensure individuals reach sexual maturity before harvest.

Existing RZs have proven to be an effective tool for allowing stocks to regenerate, but they are not currently large enough to provide sufficient spill over to replenish GUZs. Currently only 5% of PHMR is protected in RZs, whereas at least 20% is recommended by the scientific community for effective fisheries sustainability. Therefore, it is recommended that the current RZs be expanded and that new areas be identified through research and established to protect the preferred habitats of all three commercial benthic species.

The Managed Access fisheries management program is resulting in improved management of commercial fisheries in PHMR, but close monitoring and conservative management is still necessary to overcome the legacy of pre-Managed Access fishing pressure combined with natural occurrences that may impact populations of commercial species. TIDE holds regular forums with Managed Access fishermen in order to keep fishermen abreast of findings from our research and to enable TIDE to incorporate fishers’ local ecological knowledge into interpretation of results. This ensures the commercial resources of PHMR can be managed by TIDE in partnership with PHMR fishers as efficiently as possible. Accurate reporting of catch data by fishermen is critical for the success of the Managed Access program, and TIDE is continuing to work with fishermen to make sure they understand the importance and benefit to themselves of submitting accurate records.

Date: November 20, 2016 Author: dwarns
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