Week 5 trip to coast

The objective of this trip was to meet oyster growers. We had a spectacular drive from Windhoek to Henties Bay on Jan 30, newly ensconced in the Combi (VW van). We passed through a blizzard of butterflies – they seem to be bursting forth in droves just on the edge of where it’s rained. And we passed underneath a maelstrom of hawks, literally hundreds of them circling and zooming across the road, reason unknown. It was particularly surprising to find them all together since we’d commented so often on the striking absence of raptors – we’d seen less than a dozen in our travels of the previous month. We took the sand road “short cut” to bypass Swakopmund and go straight to Henties Bay, past the Spitzkoppe, a dramatic promontory that rises from the flat sandy plain in a series of sheer drops from rounded peaks. All along the road, tables were set out with rocks for sale, crystals of a variety of colors. (We have yet to visit the Crystal Museum in Swakopmund, which boasts the biggest crystal in the world, almost twice the height of a person. And of course we won’t visit the vast stretches of coast that are closed to entrance because of diamonds being collected from the sand.) On this road, we were also treated to our first experience of springbok “pronking”. They bounce, apparently straight-legged, a few meters straight up – a sign of vague unease that the Combi had stopped so the people inside could appreciate the two-dozen animals in the small herd. Springbok are excellent desert-dwellers because they never need to drink, getting sufficient water in the vegetation they eat.

One weekend morning this trip, we traveled north from Henties Bay for less than an hour to Cape Cross, which is famous for tens of thousands of Cape Fur Seals. Certainly, we’d had a fine look at them at Walvis Bay, but the Cape Cross colony was pretty amazing. The rocks were polished from generations of seals sliding across them, and the depressions in between boulders were filled with scat and carcasses, tiny skulls showing white through reddish-brown background. The smell was incredible – I actually took a shower that evening because I was convinced I could still smell seal in my hair! Also on the beach was an enormous dead sea turtle – Alan says its head was the size of a basketball and total length around 15 feet. I thought less but still impressive… and sad that such a giant is no longer roaming the oceans. The seals seemed a little more expendable, with pups rolling on and off the beach in the surfable waves – only the ones at the upper edge of the ribbon of seals along the shoreline seemed to stay put, mostly sleeping and covered in sand. The pups are darker than the adults (also, of course, smaller), so it seemed to us that most of the mothers must be off hunting for fish, or perhaps just sleeping offshore, flippers held out of the water, away from all the fuss. When mothers returned, they would bellow and haul themselves through the pack of seal bodies (adults snapping at them as they jostled and bumped), presumably looking for their own pup among the thousands there on the beach. We saw one mother very deliberately keep all pups away except one, which she let nurse.

The meetings with growers were great. We heard some familiar stories, hemispheric restatements of the problems experienced in Washington: difficult markets, mass mortalities of unknown cause, clients stolen away by other growers who undercut prices, nearby practices that exacerbate a pest problem…. The primary cultivated species is the Pacific oyster, Crassostrea gigas, which has been introduced to the US west coast, Namibia, and more than 60 other countries. As far as we have seen here, it has not established. We are puzzled by this, because the growers are complaining that the oysters are soft with gonad (spawn-y, or even spawned out), reflecting high summer water temperatures. The temperatures may be particularly high this year because, essentially since our arrival in Namibia, this high-upwelling coast has not been upwelling! The winds have blown from the northwest, pushing surface water onshore, keeping the deep cold water down deep, and allowing the warm Angola Current to push southwards to Namibia. Water temperatures have been recorded in Swakopmund and Walvis Bay around 25C (quick translation: 77F), perfectly suitable for oyster larvae to survive and grow. Yet, no spat have appeared – in Walvis Bay, the larvae may simply not be retained in this small indentation in the coast.

The methods for growing oysters differ between Namibia and Washington. The oldest oyster company (young by US standards: second generation) is actually a tiny arm of a much larger company that produces salt (and seabird guano). Seawater is pumped up several meters and then flows by gravity through a series of ponds where water evaporates, impurities (gypsum) are precipitated out, and finally sandy salt is removed in foot-cube chunks for further refining. In its initial path, the seawater is divided into two ponds: a 30-ha pond, mostly <1 m deep, has racks for growing oysters, and a slightly larger pond contains the platform to attract cormorants to breed (and defecate). Mr Klein produces his own cultchless spat by raising larvae in tiny upwellers with ambient seawater, then allowing them to recruit onto smooth plastic sheets and brushing spat off daily – both C. gigas and Ostrea edulis (the only grower on the Namibian coast to produce edulis). Larger oysters are planted into the pond in open-top mesh bags/trays, just below the water surface. Oysters are packed into the trays, but the salt pan soup of phytoplankton must be incredibly rich, fueled both by the ocean inputs and by recycled nutrients from birds. Because of the importance of these shallow-water (albeit salty) habitats for birds, the whole salt company is also a game preserve! Even with our ornithologically naïve eyes, we’ve seen 2 flamingo species, 2 cormorant species, grebes, African ducks, turnstones, at least 4 tern species, and long-legged stilt-like birds with both upturned and downturned bills.
Mr Klein described the following food web in the salt pond “chemostat” where he grows oysters: Flamingos remove most of the organisms in the sediment. Fishing birds such as terns and cormorants feed on the fish (which include “freshwater” tilapia at 40 ppm!). Fish feed on the red alga Gracilaria (deliberately introduced as a prospective aquaculture product, because it can be refined to produce agar), and perhaps also on goggas (Afrikaans, so make sure the “g’s” are raspy in your throat) – Paridotea isopods. Mr Klein observed that, coincident with his introduction of Gracilaria, a pest of his oysters disappeared: polychaete worms that bore through the shell and cause mortality and disfigurement. He wonders if the Gracilaria provided a refuge for the isopods, which then removed the worms. Is this a cool food web, or what?!??? We are likely to work on testing these interactions while we are here. Of course, there are problems, such as: 1) we haven’t been able to find the polychaete worm at all, so it may be locally extinct from the salt pond (perhaps was even introduced with oysters from Chile or the US) – not because of isopods, but because of high salinity. If the worms are no longer present, it will be difficult to test whether isopods control the worms. 2) Worldwide, boring polychaetes in oysters are controlled by dipping oysters in high-salt baths (hmmm, like a salt pond) or moving the oysters onto wooden racks off-bottom (hmmm, did some husbandry practices change at the same time the Gracilaria was introduced?). So the isopods might not be responsible for a rapid decline in worms.

Most of the oyster growers other than Mr Klein are based in Walvis Bay, where oysters are grown exclusively by suspended culture in water that tends to be less than 15 m deep. There are less than 20 companies, some of which are just a few years old. Originally, in Walvis Bay, bags were suspended from wooden platforms, but now the dominant method is “Spanish longlines”. Blue plastic 55-gallon drums are used as floats, and single (cultchless) oysters are placed in stacks of trays suspended from the floats and lines in between them. Two major sources of mortality for the oysters appear to be: low oxygen conditions lasting more than a week during sulfide eruptions – the water can be deoxygenated right up to the surface; and stressful summer conditions, involving unknown combinations of high temperatures and pathogens. Right now, as downwelling conditions persist, and the warm Angola Current pushes south, many mass mortality events have been reported, with up to 60% crop loss. Given the millions of dollars spent worldwide to understand “summer mortality”, and the general conclusion that the only remedy is to raise genetically tough oyster families, we probably won’t get deeply involved in research on this problem – no one approaches it from an ecological perspective. Fortunately, from the growers’ perspective, one company has a beautiful hatchery facility and produces millions of spat from local broodstock.