Where have all the crawfish come from?

Addressing the elephas in the room

2020 promises to be an exciting year for me. Not only am I now working on a new research project, but for the first time in over a decade in industry and research, my focus is not on lobsters… well, not ones with claws, anyway! It may not make me quite the taxonomic generalist just yet, but my new research will be working on a different species of decapod, the European spiny lobster (Palinurus elephas, or crawfish, as most fishers know them). Spiny lobsters, so called because their shell is covered in thorny spikes, are fantastic animals; they lack the large crushing claws of ‘true’ lobsters, but have enormous armoured antennae instead, and when threatened they rasp these together to create a booming noise that travels hundreds of meters under water. They are also commercially targeted as a valuable seafood commodity, being hugely sought-after by fishers who sell on to markets where they fetch vast prices. Picked crawfish tail-meat retails at a whopping £100 per kilo in the UK, and they can be even fetch double that as a live export, where single animals sell for hundreds of dollars each in the markets of the Mediterranean and Asia. This isn’t unique among the ~60 species of spiny lobster, either; just two spiny lobster species account for around 30% of Australia’s entire fishing income, and even as tiny juveniles [photo 1; right], ornate spiny lobsters in southeast Asia are worth £20 each when harvested from the wild for on-growing in netted ponds.

From Bust to Boom?

So spiny lobsters are big business but, in a familiar story across the seafood sector, where there is big demand, there is a history of overexploitation. Spiny lobsters were once caught regularly in southwestern Britain, and was part of the inshore catch of fishers working around the coast with pots and nets. However, around the 1970’s there was a pronounced decrease in abundance, largely attributed to overfishing. The commercial fishermen generally blamed divers, who they said could take too many of the adults inhabiting reefs, and it is true that the long antennae can be grabbed to catch them by hand in way that is not possible (or safe!) with clawed lobsters. The divers blamed the fishermen for increasing the scale and spread of their activities, and it is true that the proliferation of monofilament nets (and hydraulic winches to haul them) increased the intensity of fishing around this time. In reality, both activities inevitably contributed to spiny lobsters suffering a population collapse around Cornwall and Devon. There remained the occasional catch in intervening years, but the animal was rarely encountered, especially inshore – one fisheries officer glumly reported that a dozen adult spiny lobsters caught by chance in one net targeting fish off North Devon was “probably the entire remaining population of the Bristol Channel”! All the while, a few boats continued to target spiny lobsters commercially in deeper Celtic Sea waters around the Scilly Isles, but no recovery was made in the inshore waters around the mainland… until now! About 5 years ago fishermen and divers started reporting that they were seeing very small spiny lobsters around the southwest again, including inshore reefs and other areas in which they had been absent for decades [photo 2; left]. What has caused this sudden wave of new recruitment was a bit of a mystery but, since there were hardly any local adults to descend from, it seems probable that these juveniles have drifted in as larvae from established populations elsewhere.

Complete drifters

Like our clawed lobsters, crawfish start life as one of thousands of siblings hatched from the egg-clutch of a mature female, but their dispersal potential is even greater; whereas larval lobsters are thought to spend around 3-4 weeks drifting as part of the plankton, crawfish larvae (which are called ‘phyllosoma’) spend more like 4-6 months floating at sea before they metamorphose into juveniles (‘pueruli’) and settle onto the seabed. This means that populations in surrounding areas which survived throughout the period that stocks in southern England were fished out are all candidates to be the source of this new influx of juveniles. Spiny lobsters are largely absent from the North Sea, so we can rule that out as a potential source, and though they are relatively common in parts of the Mediterranean, it’s 2,000km from Malaga to Penzance, so that seems too distant to be the direct origin of our new wave of crawfish. By contrast, the Atlantic coasts of adjacent areas like Scotland and Ireland to the north, and France, Spain or even Portugal to the south have always supported spiny lobster fisheries and may not be out of reach to larvae drifting at sea for up to a year. Remarkably, considering it is only 45km west of the Cornish peninsula, there has also been sufficient abundance of spiny lobsters in the Isles of Scilly to maintain targeted fishing by a few small vessels [photo 3; right], so that is perhaps the most obvious suspect, although the Scillonian crawfish stock itself may be transient; over a hundred individuals were tagged as part of three-year study by the local conservation authority (including one by me! [photo 3; right]) but only one animal was recaptured, much fewer than the rate for clawed lobsters (14%).

A RADical Plan

Thanks to some funding from the European Maritime and Fisheries Fund (EMFF) awarded by the UK Marine Management Organisation (MMO), we have secured a project to use genetic markers to assess the origins of our newly recruited spiny lobster stock. This research will take place across four stages…

1) Obtaining tissue samples from spiny lobsters captured across the species’ range, including lots from around southwestern England, and then extract DNA from these.

2) Scan tens of thousands of sections of these lobsters’ genomes using a process called RAD sequencing, from which we can identify thousands of single nucleotide polymorphisms (SNPs), a genetic marker which varies among individuals.

3) Use the SNP markers to assess geographic differences between samples from different areas to try and detect the species’ population structure and identify the spatial boundaries between different stocks.

4) Test which of the population units the sub-adult crawfish of Devon and Cornwall are most similar to via genetic assignment, and define patterns in larval recruitment.

This analysis won’t reveal all the answers as to what has caused the crawfish recovery across southwestern shores in recent years, and a host of other factors may be at play, from improved management of nearby stocks (which may have led to more larvae being released) to warming sea temperatures (which may have improved larval survival) or prevailing currents (which may have focussed more larval drifts towards the area). A combination of these factors could be driving the settlement of all the new juveniles, or something else entirely could be influential; probably my favourite theory is that the weird, transparent phyllosoma lifestage [photo 4; left] isn’t free-drifting but instead seeks to attach to jellyfish for a free ride and a mobile food source, and that the influx of barrel jellies into the southwest in recent summers brought with them lots of crawfish passengers. But however they’re getting here, knowledge of where they have come from is important. An understanding of larval recruitment dynamics will aid conservation efforts by showing how much gene flow exists across Atlantic populations, how well different areas are connected, and the extent to which some ‘sink’ stocks may be reliant on other ‘source’ stocks for larval recruitment.

Really sticks in my Craw

Ensuring the conservation of spiny lobsters is already a huge challenge for management organisations, such as the Inshore Fisheries and Conservation Authorities (IFCAs) which regulate commercial fishing around the coastal regions of the UK. Not only do managers have to cope with the inherent uncertainty of how many crawfish will be extracted by fishermen and how many new juveniles settle, but several other factors make it hard to ensure the sustainability of their fisheries. Like other spiny lobster species, European crawfish are thought to migrate in convoy, making them vulnerable to heavy losses if these groups are all captured simultaneously. Notably, regulating the fishery by minimum landing size may also be much less effective than for clawed lobsters because of capture-induced mortality. Whereas the vast majority of undersized clawed lobsters are released unharmed from pots, most spiny lobsters are caught in nets, and are more prone to damage as these nets can cut into them as they are hauled up to the boat under pressure; researchers in the Balearic Islands of the Mediterranean have estimated that only about half of the undersized crawfish caught by fishers survive long after their subsequent release. Nevertheless, if we can discriminate between individual population units and show where greater cooperation between management organisations in different areas and countries is required to maintain larval recruitment, then this project can make an important contribution towards safeguarding all stocks of the charismatic crawfish [photo 5; below].