MARINE FISHERIES The ups and downs of rock lobster larvae

17

NIWA Water & Atmosphere 11(2) 2003

MARINE FISHERIES

The ups and downs of rock lobster larvaeJohn Booth

Steve Chiswell

Russ Bradford

Barry Bruce

The rock lobsterfishery on bothsides of the Tasmanshould benefitfrom specialisedsamplingequipment.

John Booth andSteve Chiswell arebased at NIWA inWellington. RussBradford and BarryBruce are at CSIRO,Hobart, Australia.

Two final-stagephyllosoma larvae ofthe red rock lobsterJasus edwardsii(upper) together with afinal-stage phyllosomalarva of the prawn killerIbacus alticrenatus.

The New Zealand red rock lobster Jasusedwardsii has among the longest larvaldevelopment known for any marine creature.The phyllosoma (Greek for leaf-like) larvae(below) spend close to two years in oceanicwaters before metamorphosing to thepostlarval stage, known as the puerulus, whichthen swims towards the coast to settle.Because phyllosomas are weak swimmers, theycan drift large distances during theirdevelopment. There is strong evidence, forexample, that rock lobster larvae aretransported 2000 km from southern Australiato New Zealand.Most of our lobsters, however, probably comefrom locally hatched larvae held close to theshore by features such as the Wairarapa Eddy,a large permanent eddy off the southeast coastof the North Island. Once phyllosomas are inthis eddy, they can remain trapped long enoughto reach metamorphosis. But, because the eddyvaries in position and strength, its effectivenessat retaining larvae changes from year to year.Oceanographers use a range of satellitetechnologies to identify ocean currents andeddies. We can use these data to model wherethe phyllosomas might drift, and to look at howvariations in currents and eddies might affectphyllosoma survival and dispersal. Suchmodelling could greatly benefit themanagement of the rock lobster fishery, sincebreeding females in some areas may beexpected never to contribute to larvalsettlement, while those in other areas may bemajor sources of settlement and shouldtherefore be carefully protected.

Vertical movements: joint ventureTo model phyllosoma drift, we need to knowwhere in the water column the larvae arelocated, because water movement patterns –and therefore potential drift patterns – varywith depth.Like other plankton, phyllosomas are thoughtto be nearer the surface at night than duringthe day. They form part of the “deep-scatteringlayer” of animals that rises quite rapidlytowards the surface to feed as dusk approaches.But how deep do the phyllosomas go duringthe day in avoiding predators?Since Jasus edwardsii supports extremelyimportant fisheries in both New Zealand andsouthern Australia, tracking the vertical

distribution of the larvae is of interest to bothcountries.To sample lobster larvae we use a very largenet – 60 m long – because the larvae are quitesparse in the ocean. Even in the WairarapaEddy densities rarely exceed one larva per 500m3. However, until recently we had no way ofopening or closing the net at depth in order toget an idea of vertical distributions.This was wherecollaboration withscientists fromCSIRO, Australia’snational researchorganisation, camein. CSIRO hasdeveloped a devicethat attaches to theend of the net. It hassix separate codendswhich we can openand close to obtainphyllosomas atspecific depths(right). We used thesystem successfullyin March 2003 fromNIWA's researchvessel Tangaroa ,sampling the watercolumn to at least 300m six times overseveral 24-hourperiods. We alsoretained almost allcomponents of theplankton, includingsquid, bizarre-looking fish, prawns,and salps, togetherforming a uniquebody of material for other scientists.The results so far indicate that most mid- andlate-stage phyllosoma larvae remain in theupper 50 m. There is probably only limiteddispersal to greater depths during the day.For our modelling, this result is very significantbecause we can now begin to use more realisticvertical behaviour in our models. Because midand late stage make up the most of the larvaldevelopmental period, this voyage hassignificantly enhanced our ability toreasonably model larval movements. �

Russ Bradford with the multiplecodend device secured to the endof the 60-m long fine-meshed net.

(Photos: John Booth)