Copper shark: Namibia and Angola’s shared marine predator

Copper bronzy.

Animals can be restricted and managed within man-made borders on land, but this is impossible in the open seas, says Hannes Holtzhausen, Principle Fisheries Biologist and Head: Large Pelagics in the Ministry of Fisheries and Marine Resources.

He considers the paradox that while the copper shark is protected in Namibian waters by the catch-and-release angling system, the resource is being commercially harvested in Angolan waters.

What do you do when two countries share a marine resource that ignores the international border and migrates seasonally? To compound matters further, this resource is harvested by one country and zealously protected by the other. This is the bronze whaler or copper shark (Carcharhinus brachyurus), affectionately known as bronzy in Namibia. Around the globe it has been shown that sharks cannot be harvested sustainably, as they grow very slowly, reach sexual maturity late in life and produce only a few young. These life history traits are indicative of a species that can be over-fished easily and quickly and of which a population or stock is very slow to recover, if at all.

The bronze whaler is one of the slowest growing of all shark species. Combined with its inshore habits, this makes it extremely vulnerable to over-exploitation. Preliminary tag-recapture results have indicated that Namibia and Angola share the same population of bronze whalers and that it moves seasonally between the two countries.

For recreational sport anglers in Namibia, this popular fighting fish is the ultimate prize to catch from the shore using rod and reel. Some experts put it on a par with catching marlin or sailfish – only it is much cheaper and more abundant. No wonder that this non-consumptive fishery supports an ever-growing clientele, especially of overseas visitors and also foreigners, mostly from South Africa. Angling tour operators offer their services in this way with specially designed itineraries. Overseas clients pay as much as N$25 000–N$35 000 each for a 10-day angling tour, primarily targeting bronze whalers, while foreigners pay about N$700-800 each per day to target these sharks, all of which are returned alive to sea again. Namibian Fisheries regulations stipulate that a recreational angler may retain only one shark per day, including the bronze whalers.

Also, for many years, bronze whalers were the most popular fish targeted during angling competitions in Namibia. However, local anglers have noticed a distinct reduction in their numbers over the last three years. This coincides with the large numbers being commercially harvested in Angola, specifically at Baia dos Tigres, thought to be the breeding ground and nursing area of this species. Other pelagic shark species such as thresher, blue, shortfin mako and soupfin are also harvested by pelagic longliners for their meat and fins in central and southern Angola, but because of their inshore habits, bronze whalers are easier to catch in large numbers where they are abundant, such as in Baia dos Tigres. Therefore, tonnes of bronze whalers were harvested there during 2001 and 2002.

Suspecting that harvesting this species in large numbers could harm sport fishing for it in Namibia, these concerned anglers requested the Namibian Ministry of Fisheries and Marine Resources (MFMR) to investigate the issue.

No data on size distribution, sex ratios, seasonal changes in catch rates, migration and distribution of bronze whalers in Namibia or Angola were available at the onset of the project. These parameters are currently being investigated, as well as various other subjects such as socioeconomics, genetics and so on.

Thus followed an exploratory investigation of 18 months in Namibia and Angola conducted by the Large Pelagic Section of Swakopmund, jointly funded by the MFMR, Klipbokkop Mountain Resort, Goodyear SA and the Mitsubishi divisions of Daimler/Chrysler SA. A formal project proposal was then compiled and in July 2003 funds became available through the BCLME (Benguela Current Large Ma-rine Ecosystem) programme, to conduct a three-year project on the migratory behaviour and assessment of the bronze-whaler resource.

This project (LMR/CF/03/16) is now a registered BCLME project of the MFMR. The funds are from the World Bank and are channelled through and administered by BENEFIT (Benguela Environment Fish-eries Interaction & Training) Programme.

The backbone of this pro-ject is a tag-and-release programme and here private anglers, angling tour operators and angling clubs are assisting the researchers. A yellow plastic dart tag is inserted into the shark’s flesh with a punch-like applicator next to its dorsal fin. The tag has a numerical code and number which is unique to each individual. To date some 1 000 bronze whalers have been tagged and released, of which 38 recaptures have been recorded. These results indicate that at the onset of summer, adults of this species migrate southwards into Namibian waters as water temperatures in Angola in-crease. The sharks then stay in the central coastal area for the remainder of summer and then start migrating back to Angola at the onset of winter in May. Biological data such as lengths, population structure, weights and sex ratios are also collected while tagging the sharks. All these parameters will eventually be used to model the state of this shared resource.

In addition, for the first time in Namibia, four archival pop-up satellite tags have been deployed on bronze whalers. These tags can be pre-programmed to release from the shark at a specified time and can function for up to a year. They pop up to the surface and beam the shark’s location and various other data to an orbiting satellite. Conventional tags show only the points of release and recapture and not how the shark has behaved in between. Thus, some of the bronze whalers tagged and recaptured in Namibia could have migrated to Angola or elsewhere and back. Pop-up tags capture and store the exact route the shark has followed during the time it has been carrying the tag. Hopefully this will give us valuable data in a short time.

The main objective of this project is, once the necessary data has been collected by the end of a three-year period, to formulate a joint management plan for the bronze-whaler resource. As scientists from both countries are involved, the project strengthens re-search and human capacity building. Joint management will ensure that a straddling stock such as this will have a safe future while being utilised on a sustainable basis.

This article appeared in the 2004/5 edition of Conservation and the Environment in Namibia.