Baby (Smith’s swimming) crabs

Last week we met Smith’s swimming crab (Charybdis smithii), a rare visitor from the tropics that arrived in the southern reaches of False Bay during the third week of January 2019. We found some adult individuals at Long Beach, well inside False Bay, on 3 March.

Juvenile Smith's swimming crabs at Kommetjie
Juvenile Smith’s swimming crabs at Kommetjie

At the same time that fishermen were reporting adult Smith’s swimming crab off Cape Point, a huge number of tiny red crabs washed up on Long Beach, Kommetjie. There were great piles of them, but we only managed to visit a day or two later on Friday 18 January.

Juvenile Smith's swimming crab at Kommetjie
Juvenile Smith’s swimming crab at Kommetjie

We had to hunt a little, but after a while we found the crabs – now only a few remaining, but most of them very active and vital. Opinion from George Branch is that these are most likely the final larval stage of Smith’s swimming crab, and the same warm conditions wit anomalous westerlies that brought their adults to the peninsula, brought the juveniles.

Juvenile Smith's swimming crab at Kommetjie
Juvenile Smith’s swimming crab at Kommetjie

This conversation (facebook group, may be closed) suggests that the juvenile Smith’s swimming crabs were also seen on Milnerton Beach around the same time as they appeared on the beach at Kommetjie.

I took a couple of short videos of the tiny crabs, which show them in motion on the wet sand.

Apologies for the loud wave noise and less than perfectly steady camera work. Read more about these pelagic crabs, exceptionally infrequent visitors to our shores, in this post.

Hello, Smith’s swimming crab!

One of the things I love about False Bay, and about Long Beach in particular, is the tendency for creatures from all over South Africa’s coastline to end up here, often tropical marine life that got caught in the warm Agulhas current, and then within the circulation of False Bay, ending up just behind the Simons Town harbour wall.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

Thus it was, early in March, that we discovered several adult specimens of Smith’s swimming crab (Charybdis smithii) at Long Beach when we went for a dive. One or two were already dead, and the rest were struggling either on the sand, or in the shallows, looking unhealthy. The water temperature at the time was about 17 degrees.

The first hint that this unusual and rarely-seen visitor had arrived on our shores was a series of social media posts, from January, in one or two of the fishing groups I follow on facebook. (These are excellent places to keep tabs on what’s happening in parts of the ocean I might not routinely visit, and there’s a wealth of knowledge and experience among the members.) Here’s a conversation between local fishermen about seeing large numbers of adult Smith’s swimming crabs just off Cape Point (also facebook). You can also see some photos of one of the crabs from Sea-Change here (facebook), taken on 22 January in False Bay.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

At the same time as these social media posts, there was an influx of small, red crabs on the other side of the Cape Peninsula, at Long Beach in Kommetjie. (But more on that in another post.) Two Oceans says that Smith’s swimming crabs were first described in False Bay in 1838, and then again in 1978, 1983, and 1993. This facebook thread suggests that they may have been last seen off Muizenberg around 2005-2006.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

I read more about Smith’s swimming crab in two papers: this one (Romanov et al), from 2009, and this one (Van Couwelaar et al) from 1997. The more recent paper updates many of the findings of the earlier one. Both teams of scientists behind these papers used trawl data from pelagic cruises to learn about the distribution and life history of these crabs.

Smith’s swimming crab is a pelagic crab that spends the vast majority of its one year, monsoon-driven life cycle in the water column. They are endemic to the western Indian ocean, and are usually found in the area bounded by the Arabian sea (which is west of India) and the latitude of Dar Es Salaam in Tanzania (about 7 degrees south of the equator), and from the east African coast, across east to the Maldives. They may congregate in huge patches, larger than tens of square kilometres, and may reach a biomass of more than 130 kilograms per square kilometre. These swarms are densest between June and September. During July, their concentration can peak at more than 15,000 individuals per square kilometre.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

The crabs congregate on the seafloor of the continental shelf during the months of September to December, mating late in the year. No adult crabs are usually seen between April and June (Van Couwelaar et al speculate that the adults die after breeding), at which time, after metamorphosis, the swarms again become apparent in the western Indian ocean. The crabs grow to about 7.5 centimetres carapace width.

The crabs seem to perform a diel migration, moving deeper in the water column during the day (down to 350 metres’ depth), and returning to the surface at night. They swim continuously and are voracious predators in order to support the high metabolic demand created by this constant activity. They are able to regenerate all their limbs except for their swimming legs (Van Couwelaar et al deduced this in much the same way as Abraham Wald decided which parts of World War II bombers to reinforce – no crabs with partially grown swimming legs were caught in their trawls).

These crabs are important prey for yellowtail, as for other pelagic fish species such as blue sharks, yellowfin tuna and bigeye tuna. The fishermen of False Bay observed that they made excellent bait.

Smith's swimming crab (Charybdis smithii) Smith’s swimming crab (Charybdis smithii)

So what brings Smith’s swimming crab this far south? False Bay is way out of their range. This paper (Chapman, 1988) suggests that prior arrivals of these crabs on our shores have co-incided with weaker than usual summer south easterly winds (which has definitely been a feature of late 2018 and the start of 2019 – we had a gloriously wind-still summer for the most part) and the westward movement of warm water containing the crabs. We did have a spell of unusual westerly wind just prior to these crabs’ arrival.

A fascinating 1984 paper by George Branch describes a temperature anomaly during the summer of 1982-83. This particular Cape summer was characterised by very little of the typical south easterly winds, leading to reduced upwelling, and relatively high sea temperatures (Duffy et al, Effects of the 1982-3 Warm Water Event on the Breeding of South African Seabirds, 1984). The exceptionally warm water on the south and west coasts of South Africa caused mass strandings of some tropical animals (such as portuguese man ‘o war), mortalities of others (such as black mussels), changes in abundance of some species, and extensions of some species’ geographic range. For example, an exceptional number of juvenile turtles washed up on the beaches of False Bay, several months before the usual start of the usual turtle stranding season (which is, very loosely speaking, March-July). Prof Branch records that large numbers of healthy, adult Smith’s swimming crab washed up at Cape Hangklip, and smaller numbers at Boulders Beach, Strand, Milnerton and Blouberg. About 62% of the crabs were female, and many of them survived in aquaria for some time after stranding.

In short, it seems that we had our own little temperature anomlay, however brief, in early 2019, and the pulse of warm water brought with it these rarely seen (in Cape Town) crabs. What luck to spot this unusual visitor!

There are some lovely pictures of Smith’s swimming crab, healthy and in mid water, taken off Tanzania, here.

Shrimp news from False Bay

The University of Cape Town has announced that a further three new species of shrimp, all spotted close to shore near Millers Point in False Bay, have been described and named. All three belong to the same genus (Heteromysis), and look similar, with pale bodies marked by red spots and stripes. One of these new (to science) species lives inside octopus dens, and another lives inside the shell of certain types of hermit crab. These three shrimps join the stargazer shrimp that was discovered by and named for Guido Zsilavecz, citizen scientist and author of several books on False Bay’s marine wildlife.

Two of the new species were discovered by local film maker Craig Foster, founder of the Sea-Change project about which we read last week. These types of discoveries are very exciting and should be a great inspiration and encouragement to divers and other water users. Time in the water is rewarded. If you can’t identify something, send an email with its photo to SURG. It is possible to make significant contributions to science while holding down an entirely non-scientific day job!

Read all about the new shrimps here.

Newsletter: Try it out

Hi divers

Weekend dive plans

Saturday: 6.00 am on Simon’s Town jetty for double tank dives in the Roman Rock vicinity

Saturday: Dives from Simon’s Town jetty at 9.00 and 11.30 am, sites dependent on conditions

Dive conditions

We had good conditions last week; they held for the weekend and then kept going at the start of this week. Yesterday and today, however, the viz took a bit of a nosedive and is possibly going to settle into the summer visibility groove of a warm 4- 6 metres, depending on your eyesight. There is very little swell or wind in the forecast which will help.

I doubt there will be too much difference between Saturday and Sunday so the plan is as follows: a screechingly early double tank launch on Saturday (6.00 am on the Simon’s Town jetty). On Sunday we will meet for 9.00 am and 11.30 am. The sites will depend on what we find on Saturday.

A Cape long-legged spider crab
A Cape long-legged spider crab

This Cape long-legged spider crab hitched a ride to the surface on one of the divers’ booties this week. Isn’t he a handsome chap? He is back where he belongs!

Try diving in the pool

In the month of December until Christmas, we are offering Discover Scuba sessions (try dives) in our pool, free of charge, every Wednesday and Thursday after 3.00 pm. If you have a friend that needs a little persuasion to qualify as your future dive buddy, then bring them along. Booking is essential. Get in touch if you want to reserve a slot.

regards

Tony Lindeque
076 817 1099
www.learntodivetoday.co.za
www.learntodivetoday.co.za/blog/

Diving is addictive!

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Series: Deadliest Catch, Season 8

Deadliest Catch, Season 8
Deadliest Catch, Season 8

It’s been a while since we’ve watched any Deadliest Catch, but I needed to feel good about my job again, so a dose of this Discovery Channel favourite was required.

There are a couple of younger captains this season; we were interested to see how much they talked up their abilities, and how often they took ridiculous risks that endangered their crew. One of them in particular brought his train wreck of a personal life on board at the start of the season, which culminated in a restraining order from his ex girlfriend and almost no pay for his crew.

We also found the family dynamics interesting. There are many fathers and sons, and pairs of brothers, working in the fleet, and family feeling is strong. Some of the relationships are deeply unpleasant. The captain of the Kodiak, who demonstrates repeatedly what a mean, vindictive person he is, repeatedly belittles his son after losing patience with him when he doesn’t learn fast enough. By contrast, the supportive learning environment provided on board the Northwestern enables some of the young crew to grow and learn. Edgar, one of our favourite people in the show, gets an opportunity to spend some time in the wheelhouse when his brother Sig (who is an awe-inspiringly consistent fisherman) lets him take a turn at driving the boat.

One of the boats featured in the series had a crew member medevaced as a result of a panic attack (the official line was “seizure disorder”, but he was clearly having a panic). This hasn’t happened before – there is always a feature with the US Coastguard, but it has always involved other boats up to this point. The calm of the helicopter pilots and rescue swimmers is very impressive.

The latter part of this season had a soap opera feel to it. An ice-induced hiatus in the opilio crab season led to many of the captains taking a break from fishing, to wait for the ice to clear from the crab grounds. The film crew follows the captains and crew home, and – in some cases – deep into their personal problems during this period. Tony and I were relieved when they got back to fishing, although it was nice to see the beautiful homes, mostly in Washington State, that the big bucks earned crab fishing can buy.

Did this help me feel better about my job? Yes it did: sitting at my desk in a winter sunbeam isn’t too bad, or too difficult. The chances of losing the end of one of my fingers or being struck on my head by a chunk of ice are slim. Small mercies.

Series: Deadliest Catch – Lobster Wars

Lobster Wars
Lobster Wars

Lobster Wars is a Discovery Channel production, produced by the same team who brought us Deadliest Catch (seasons 1, 2, 3, 4, 5, 6, 7) and its Tuna Wranglers spin-off. It tracks fishermen (and a woman) on board the American lobster boats that set out to fish Georges Bank from the beautiful New England harbours (and expensive holiday destinations) in Massachusetts and Rhode Island.

This is a slightly tamer version of Deadliest Catch. The fishermen work long, hard hours on occasion, but the labour is not as backbreaking as it is on board a crab boat. They are at sea for a week at a time, and the lobster traps are lighter and smaller than the crab pots seen on Deadliest Catch. The boats are small, and the fishery is a low volume, high value one – two or three lobsters in a trap is worth getting excited about.

Lobstering can be extremely lucrative, particularly during the winter season shown in the episodes of this series. Fierce competition on the fishing grounds and heavy fishing pressure on a valuable resource (which can sometimes be in oversupply) makes for a turbulent working environment – never mind the weather. While not quite as stormy as the Bering Sea, Georges Bank can throw up some extreme weather events of its own.

The fishing grounds on Georges Bank are “controlled” by different boat captains, who have time-tested locations that they return to year after year. We found this puzzling – that one could exert control at a distance over a piece of sea floor with relatively few conflicts. Or perhaps not so few – this article explains the phenomenon quite well. One source of conflict that recurred repeatedly in this series was between the lobster boats and trawlers, called “draggers” by the lobstermen. The trawlers shown working Georges Bank had outriggers, and if a string of lobster traps gets caught in their gear, the traps can be dragged for miles, and left in a tangled heap far from their original location.

The antics of the crewmen are mildly entertaining, but we struggled to differentiate them because of an apparently universal fondness for pulled-up hoodies among lobstermen. One female crew member is featured, working on board a boat called the Timothy Michael, and acquitting herself marvellously. A new crewman exclaims in disbelief that there’s a woman on board, commenting that he’s been on a boat where there’s been a dog on board, but never a woman. I was impressed by his liberal attitude, and am sure he’s in a supportive, mature relationship with an incredible human being who values his unique strengths and abilities.

This isn’t Deadliest Catch or Tuna Wranglers, but it is entertaining enough. The scenery, of New England and the seascapes, is lovely, and learning about a new fishery is always interesting. There are the usual lyrical waxings about how the “fishery is dying”, but the problem isn’t examined further, and no one dares to suggest that perhaps we’ve already eaten most of the fish in the sea, and if we carry on at this pace, we’ll eat it all.

You can get the DVD on Amazon.com.

Dive sites: Brunswick

Tony and students on the surface over the wreck of the Brunswick
Tony and students on the surface over the wreck of the Brunswick

The Brunswick is a historical wooden shipwreck that lies a few hundred metres off the northern end of Long Beach in Simon’s Town, directly opposite the northern end of the white apartment buildings overlooking the Main Road. Like HNMS Bato, she is infrequently dived. Having lain underwater since 1805, she is heavily overgrown and much of her decking and hull is covered by sand. She used to be a shore entry (with a precipitous climb over the railway line), but in recent years a large number of boulders have been added as a breakwater between the ocean and the railway line, and climbing over in dive gear is no longer possible. For this reason we do the dive from the boat. Close to shore and in shallow water, the Brunswick is an ideal site to get used to boat diving.

Extensive field of wooden decking
Extensive field of wooden decking

The Brunswick was a British East Indiaman, which means she carried men and goods between Britain and the East Indies – (south)east Asia and India. She was carrying a cargo of cotton and sandalwood from China back to Britain when she was captured by some French vessels off Sri Lanka, and brought to Simon’s Bay. In September 1805 her anchor rope parted, and she ran aground during a south easterly gale. Most of her cargo was salvaged, as she lies in shallow (less than six metres deep) water.

We found the dive site to be similar to HNMS Bato, which was also a sturdily built wooden ship of similar vintage. The Brunswick was 1,200 tons, and her wreckage is spread out quite extensively. There are many thick, wooden planks, laid out as they would have been to form her decks, as well as much evidence of the bronze bolts that secured parts of the ship together. There are also many copper bolts, rivets and what could be small amounts of rolled up copper sheathing in evidence on the site.

Anemone among feather stars and papery burnupena
Anemone among feather stars and papery burnupena

The highests parts of the wreck are covered with feather stars, anemones, sea cucumbers, and kelp. There are many octopus, and peering under the wreckage with a torch yielded a couple of very large pyjama catsharks. We were lucky to dive the site most recently on a day with lovely visibility, and the shallowness of the water means that there’s a lot of light penetration which improves things enormously.

The highest parts of the Brunswick wreck
The highest parts of the Brunswick wreck

Before diving this site, you should call the SA Navy Ops Room on 021 787 3818, to ask for permission and to tell them how long you’ll be. Same procedure as at Long Beach.

Dive date: 13 July 2013

Air temperature: 19 degrees

Water temperature: 15 degrees

Maximum depth: 5.4 metres

Visibility: 10 metres

Dive duration: 42 minutes

Mark helps Christo at the boat after the dive
Mark helps Christo at the boat after the dive

Series: Deadliest Catch, Season 7

Deadliest Catch, Season 7
Deadliest Catch, Season 7

Deadliest Catch is a long-running reality television series aired on the Discovery Channel. We watch it on DVD. The seventh season once again reminded us how fortunate we are to have jobs with at least some concept of “office hours”, and safe, mostly predictable working conditions. Season 6 was marked by the death of one of the captains, Phil Harris, whose two young sons, Jake and Josh, are left as part-owners of the vessel Cornelia Marie. The conflicts inherent in being both a boat owner and a deckhand are shown in technicolour as they clash with the captain they hire for the red king crab/blue crab season, and after catching no crab in a week, they head back to port. Their opilio season is a lot more successful, and the captain they hire for this fishing stint spends time teaching them and gives them opportunities to drive the boat and make decisions about where to fish.

A similar spirit of mentorship is seen on the Northwestern, where Captain Sig invests considerable time and effort into one of his young deckhands, also called Jake. His brother Edgar, erstwhile deck boss, has left the ship, but we were surprised how well things continued to run without his laconic presence. Jake, who is occasionally prone to bizarre immature outbursts (“I hate you all!” is a frequent refrain) is groomed to one day run the ship – he has been on the Northwestern for several years – and Sig somehow manages to strike the right mix of encouragement and discipline, giving Jake more and more responsibility for the day to day running of the deck, allowing him to set pots, and to drive the boat on occasion.

Two new boats feature in this season, both captained by young men (one still in his late twenties) with a good deal of attitude and cockiness. One of them (Captain Scott “Junior” Campbell) makes a really good showing, and we were impressed by how he managed his layabout younger brother. His calm demeanour was in stark contrast to Captain Keith Colburn of the Wizard, whose anger management problems even vented themselves on one of the show’s cameramen who happened to be walking past when the fishing was poor.

Other than the two new boats, this season is more of the same, and if you enjoyed the prior seasons I can recommend this one. We did appreciate a few more underwater shots and what looked like footage captured by attaching a GoPro to the hook that the crew use to retrieve the line attached to the submerged crab pots. There isn’t much from the US Coastguard, but there are truly awesome Arctic storms and some very gory (but not serious) injuries. I expect most people will wince the most at the sight of Captain Scott struggling with a kidney stone!

There’s a very interesting interview with the impressive captain of the Northwestern, Sig Hansen, here.

You can buy the DVDs here if you’re in South Africa, and here if you’re not.

 

Friday poem: A Few Lines from Rehoboth Beach

Via Beach Chair Scientist. Fleda Brown is a contemporary American poet.

A Few Lines from Rehoboth Beach – Fleda Brown

Dear friend, you were right: the smell of fish and foam
and algae makes one green smell together. It clears
my head. It empties me enough to fit down in my own

skin for a while, singleminded as a surfer. The first
day here, there was nobody, from one distance
to the other. Rain rose from the waves like steam,

dark lifted off the dark. All I could think of
were hymns, all I knew the words to: the oldest
motions tuning up in me. There was a horseshoe crab

shell, a translucent egg sack, a log of a tired jetty,
and another, and another. I walked miles, holding
my suffering deeply and courteously, as if I were holding

a package for somebody else who would come back
like sunlight. In the morning, the boardwalk opened
wide and white with sun, gulls on one leg in the slicks.

Cold waves, cold air, and people out in heavy coats,
arm in arm along the sheen of waves. A single boy
in shorts rode his skimboard out thigh-high, making

intricate moves across the March ice-water. I thought
he must be painfully cold, but, I hear you say, he had
all the world emptied, to practice his smooth stand.

Bookshelf: The Edge of the Sea

The Edge of the Sea – Rachel Carson

The Edge of the Sea
The Edge of the Sea

The Edge of the Sea completes the trilogy begun by Under the Sea-Wind and The Sea Around Us. Its focus is on the coastline, the meeting point between land and ocean where one is most conscious of the passage of time and the cycles of nature. Rachel Carson was an American writer, and lived and worked on the east coast of the United States. The northern reaches of this coast are similar to the Cape Town coastline, in that there are kelp forests and much invertebrate life in the cool water. Its southern reaches in the Florida Keys, however, are characterised by coral reefs and mangrove forests, more reminiscent of Sodwana Bay in KwaZulu Natal, on South Africa’s north western coast.

This is a more conventional piece of nature writing than Under the Sea-Wind, and its scope is far narrower than The Sea Around Us. It is fascinating, however, to delve into the secret lives of crabs, sand fleas, limpets, urchins, sea stars, clams and a large number of their neighbours. While – for the most part – we don’t find the exact same creatures on our side of the Atlantic, their adaptations to life in the intertidal zone are similar, and their behaviour and diet is too.

The section on the coral reefs and mangroves of Florida was interesting to me because that kind of coastline is relatively unfamiliar – I’ve only visited coral reefs three times (Zanzibar, Sodwana twice) and don’t know nearly as much about how those ecosystems work. It was the first time someone articulated for me a point that – in retrospect – is probably completely obvious to everyone else on earth, but for me was a lightbulb moment. Coral reefs only occur on east-facing coasts (think about the location of the Florida Keys, the Great Barrier reef in Australia, the east African coral reefs), as the western coasts of continents are typically subject to upwelling driven by wind and the direction of the earth’s rotation.

Rather than being an active participant in the life of the shore, man is portrayed here as an observer, unable to influence the tidal and seasonal rhythms that drive all behaviour here. The book is illustrated with beautiful line drawings and one or two maps, and I’d recommend it highly. There’s a comprehensive glossary with full species names at the back.

You can buy the book here, or for Kindle get it here.