Early warn­ing is key to en­sur­ing in­dus­try can re­spond to the grow­ing threat of HABs

Fish Farmer - - Contents Editor’s Welcome - BY MAEVE EA­SON HUB­BARD

HABs and HAB nots

AL­GAL blooms of­ten hit the head­lines these days, fea­tur­ing in more than 500 news re­ports last year in the US alone. This in­tense me­dia fo­cus re­flects the fact that al­gal blooms are now oc­cur­ring more of­ten, at greater in­ten­si­ties and across a wider ge­o­graphic range than ever recorded be­fore.

A pa­per re­cently pub­lished in the jour­nal Na­ture probed the chang­ing dy­nam­ics of fresh­wa­ter al­gal blooms us­ing three decades of global satel­lite data. Their find­ings were un­equiv­o­cal – the in­ten­sity of sum­mer­time blooms has in­creased in more than two thirds of lakes ex­am­ined world­wide.

Sim­i­lar trends are seen in the marine en­vi­ron­ment. In 2013, a green al­gal bloom formed on the coast of China that broke all pre­vi­ous size records in the re­gion, ex­tend­ing an as­ton­ish­ing 28,900 square kilo­me­tres.

Al­gae are a di­verse group of aquatic, pho­to­syn­thetic or­gan­isms that range from mi­cro­scopic, sin­gle-celled, phy­to­plank­ton to large multi-cel­lu­lar sea­weeds. When the den­sity of an al­gal pop­u­la­tion increases rapidly, it is de­scribed as a bloom.

Blooms can oc­cur nat­u­rally, driven by a com­plex in­ter­play of fac­tors such as the avail­abil­ity of nutri­ents and light, wa­ter tem­per­a­ture and cur­rents.

The rise in al­gal blooms over the past sev­eral decades, how­ever, likely re­flects an­thro­pogenic changes to the en­vi­ron­ment, such as nu­tri­ent pol­lu­tion, cli­mate change and the in­creased trans­port of al­gae in the bal­last wa­ter of ships.

Al­gal blooms that have an ad­verse ef­fect on the aquatic ecosys­tem, econ­omy or hu­man health are de­scribed as ‘harm­ful al­gal blooms’ or HABs.

The type of harm caused by HABs can vary. Some HAB form­ing mi­croal­gae pro­duce tox­ins that kill nearby an­i­mals or bio-ac­cu­mu­late within them, caus­ing ill­nesses such as par­a­lytic shell­fish poi­son­ing (PSP) in hu­mans if eaten.

Oth­ers are non-toxic and in­stead in­flict phys­i­cal harm. The HAB form­ing mi­croalga Chaeto­ceros, for in­stance, bears a set of for­mi­da­ble sil­ica spines that can dam­age the gills of fish, in some cases lead­ing to suf­fo­ca­tion.

Many HABs al­ter the en­vi­ron­ment around them so se­verely that it be­comes un­in­hab­it­able for other or­gan­isms. The de­cay of dead al­gae within a bloom, for

“There are var­i­ous rea­sons why a satel­lite is handy but not the be all and end all”

ex­am­ple, can rapidly re­duce the avail­abil­ity of dis­solved oxy­gen in the wa­ter, cre­at­ing hy­poxic ‘dead zones’ that are un­able to sus­tain fish and other aquatic life.

Such harm­ful al­gal blooms can, of course, in­flict large scale fi­nan­cial losses on the aqua­cul­ture in­dus­try. In­deed, farmed fish are par­tic­u­larly at risk as they are un­able to avoid the lo­ca­tion of a bloom as a wild fish might.

In 2016, a bloom of Pseu­dochat­tonella killed ap­prox­i­mately 15 per cent of Chile’s an­nual sal­mon stock, worth an es­ti­mated US$800 mil­lion.

To min­imise the risk of such dev­as­tat­ing fish kills in the fu­ture, ef­forts are be­ing made to im­prove the tool­kit avail­able to mon­i­tor, pre­dict and mit­i­gate the oc­cur­rence of harm­ful al­gal blooms.

HAB mon­i­tor­ing pro­grammes of­ten in­volve the rou­tine col­lec­tion of wa­ter sam­ples to be an­a­lysed in the lab­o­ra­tory.

Tra­di­tional meth­ods for de­tect­ing harm­ful al­gae in these sam­ples (for ex­am­ple, mi­croscopy, im­muno­log­i­cal screen­ing for tox­ins) have more re­cently been com­ple­mented with a suite of highly sen­si­tive molec­u­lar probes and as­says that are ca­pa­ble of iden­ti­fy­ing in­di­vid­ual HAB species at very low den­si­ties.

Out­side the lab­o­ra­tory, a range of in situ mon­i­tor­ing tools are also be­ing de­vel­oped. A Scot­tish

con­sor­tium, for ex­am­ple, is cur­rently work­ing on an op­ti­cal sen­sor sys­tem that can be de­ployed at aqua­cul­ture sites to de­tect the pres­ence of harm­ful al­gae.

In a re­cent press re­lease, Chris Hyde, chief com­mer­cial of­fi­cer at con­sor­tium mem­ber OTAQ, said that this new tech­nol­ogy ‘will fun­da­men­tally au­to­mate the (de­tec­tion) process and pro­vide ac­cu­rate in­for­ma­tion about plank­ton num­bers 24 hours a day’.

Re­mote sens­ing via satel­lite has also emerged as a valu­able tool for mon­i­tor­ing harm­ful al­gal blooms. This form of data col­lec­tion may of­fer sev­eral ad­van­tages over lab­o­ra­tory based meth­ods, as it can oc­cur in real-time, is less labour in­ten­sive and can be con­ducted on a larger scale.

How­ever, Pro­fes­sor Keith David­son of the Scot­tish Associatio­n for Marine Sci­ence (SAMS) ex­plains that re­mote sens­ing via satel­lite should not be re­garded as a sil­ver bul­let for bloom mon­i­tor­ing ei­ther, as some al­gae (par­tic­u­larly toxin pro­duc­ing species) are harm­ful at much lower den­si­ties than can be de­tected by satel­lite.

‘A harm­ful al­gal bloom can con­sist of just a few hun­dred cells per litre, de­pend­ing on the species,’ said Pro­fes­sor David­son. ‘But to de­tect a bloom by satel­lite, you’d prob­a­bly need ten thou­sand or a hun­dred thou­sand cells per litre.’

Satel­lites are also lim­ited in their abil­ity to de­tect al­gae lo­cated be­low the sea sur­face.

‘If there is strat­i­fi­ca­tion of the wa­ter col­umn, you might get a thin layer of al­gae sit­ting at 10m depth, for ex­am­ple, which a satel­lite won’t be able to de­tect. So there are var­i­ous rea­sons why a satel­lite is handy but not the be all and end all.’

Pro­fes­sor David­son and col­leagues at SAMS have col­lab­o­rated with part­ner in­sti­tu­tions across Europe on two suc­ces­sive projects (ASIMUTH, and now PRIM­ROSE) to de­velop and re­fine a sys­tem for fore­cast­ing HABs that threaten the aqua­cul­ture in­dus­try across the At­lantic Arc in Europe.

Fore­casts are gen­er­ated by in­cor­po­rat­ing data from HAB mon­i­tor­ing pro­grammes (for ex­am­ple, mi­croscopy counts and tox­i­c­ity lev­els) and satel­lite re­mote sens­ing into math­e­mat­i­cal mod­els of phys­i­cal and bi­o­log­i­cal pro­cesses.

The team at SAMS pre­pare HAB fore­casts for Scot­land, which are shared with users on­line (https://www.habre­ along­side re­gional HAB mon­i­tor­ing data.

Pro­fes­sor David­son de­scribes these HAB fore­casts as ‘sim­i­lar to a weather fore­cast, in that it gives a pic­ture of what’s com­ing up in the next week or so’.

This early warn­ing can pro­vide the aqua­cul­ture in­dus­try with a valu­able op­por­tu­nity to mit­i­gate the im­pact of a bloom, through ac­tions such as the re­lo­ca­tion of en­clo­sures or early har­vest­ing.

Look­ing for­ward, the ex­pan­sion of such early warn­ing sys­tems be­yond Europe could be key to en­sur­ing that the global aqua­cul­ture in­dus­try can re­spond to the grow­ing threat posed by harm­ful al­gal blooms in a timely and ef­fec­tive man­ner.

Be­low: This RGB com­pos­ite im­age from Sen­tinel-2A taken on 7 Au­gust 2015 has a spa­tial res­o­lu­tion of 10m. It shows an al­gal bloom in the cen­tral Baltic Sea.

Above: Mus­sels on Loch Fyne. Far left: Queen scal­lops. Op­po­site Im­age of pseudo-nitzschia se­ri­ata com­plex- a type of di­atom re­spon­si­ble for pro­duc­ing do­moic acid which if con­sumed via in­tox­i­cated shell­fish will cause am­nesic shell­fish poi­son­ing (ASP). They can form chains in ex­cess of 20 cells long. (Pho­tos: SAMS)

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