Blended microalgae feeds improve growth rates, survival of young geoduck clams

Microalgae blends boost growth, survival in farmed geoduck clams, says researcher. 

A group of Canadian researchers sought to answer the question of what strains of microalgae could offer the best diet for farmed Pacific geoduck clams, said corresponding researcher Chris Pearce, with Fisheries and Oceans Canada at the Pacific Biological Station.  

“Hatcheries usually use at least two if not more [microalgae strains], but in the past it’s kind of hit or miss and based on previous experience, not based on scientific research,”​ he told FeedNavigator. “The hatchery would say, ‘Last year we tried these three, and it didn’t work that great.’”​

“They play around with it over time, but not usually with much scientific research,” ​he said. “But it’s generally recognized that one species is probably not going to cut it.”​

The team tested eight varieties for use in production of the farmed clams at larval and juvenile stages, including “two prymnesiophytes, Pavlova lutheri (PL) and Tisochrysis lutea (TL); two chlorophytes, Dunaliella tertiolecta (DT) and Tetraselmis suecica (TS); and four diatoms, Chaetoceros calcitrans (CC), Chaetoceros muelleri (CM), Phaeodactylum tricornutum (PT), and Thalassiosira pseudonana (TP),”​ said the researchers in their report. In addition to looking at clam growth and survival weight, diets were assessed for fatty acid ratios and EPA/DHA ratio.

The group found the best results with a dual combination of CC and TL for the larvae and CM and TL for juveniles, said Pearce.

Diets with several strains also offered positive results, but they increased diet complexity, he said. “The fewer phytoplankton species you have to culture the better in terms of time, labor and cost,”​ he added.

Why microalgae?​

Geoduck clams are farmed in multiple locations through the US Pacific Northwest and in British Columbia in Canada, said the researchers. Farmed clams generated about 612.9 tons or $18.5m in Washington state in 2010.

However, in British Columbia producers continue to see high rates of larval mortality, they said. More information is needed on the use of different phytoplankton in the diets of young clams and optimal feeds have not been established.

The lack of information on early life stages has hindered hatchery production, they said. However, work on diets for other bivalve species has examined the role of polyunsaturated fatty acids (PUFAs) including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)

The strains checked were not indigenous to the area, but have seen long-term use in geoduck production and aquaculture diets, said Pearce. “They are common species that are used in the aquaculture of bivalves they’re kind of the tested and true species that people know,”​ he added.

“The hatcheries had been struggling for a number of years,” ​he said. “No one really knew what was happening, but we suspected that it was diet related.”​

There are also other factors involved in clam survival, like ocean acidification or fungal challenges, he said.

Study details​

In the feeding study, both the larvae and juvenile clams were evaluated in a three phase process checking different diets, said the researchers.

Larvae clams were stocked at a density of three individuals per ml-1 fed for 26 days, initial phase diets used mono-species of the microalgae, said the researchers. A diet blending equal portions of all eight strains was also tested.

At the end of the trial, containers were collected and average shell length and survival rate were calculated, they said.

In the second phase of the study, the individual strains with the best results, PL, CC and CM were combined with TL into all possible two, three and four-strain diets, they said. “Each mixed-algal diet tested comprised 50% (by AFDW [ash-free dry weight]) of TL and 50% of the other species (i.e. by mixing equal AFDW of the other component species, totaling 50% of the AFDW of the diet),”​ they added.

A mono-species diet of TL was used as a control, they said. And other conditions were the same as the first trial.

The third phase tested the TL and CC diet at 80% AFDW with the four rejected strains and followed the established produces, they said.

 Juvenile clams were also tested initially with the eight individual strains and a diet combining all types of microalgae, said the researchers. Eighty clams were fed the diets for 28 days.

Second phase testing mixed the CM strain with CC, PL and TL in combinations of two, three and four strain diets were fed, they said. Shell length was measured on days 10, 18 and 28 and animals were harvested for biochemical analysis on day 28.

The third phase examined the CM and TL diet at 80% AFDW with 20% of one of the untested strains, DT, TS, PT or TP, they said.

Findings ​

For the larval clams, growth was best supported for the eight-strain mix, TL, CC, PL and CM, although CM clams initially grew more slowly and mortality rates were lower for those getting PL and CC, said the researchers. Growth was slowest for those fed DT, TS, PT and TP and most had died by day 20.

In the second phase, the largest clams were fed the TL and CC diet, they said. Survival was 0.2-3% for all diets as a suspected fungal infection occurred.

In the third phase results were similar for all of the diets tested, said the researchers. Survival ranged from 6.4-21.3%.

For the juveniles, initial diets saw the best growth and dry-weight averages for those getting TL, CM, PL and CC diets, said the researchers. Those getting the CM diet also had the best survival rate at 53.5%.

In the second phase, clams getting the CM and TL diet had the best shell-length and dry-weight averages, said the researchers. Results from all feed tested in the third phase were similar.

“What we think is more likely happening is that you have a suite of dietary components that the larvae or juveniles need,” ​said Pearce. It takes two or three of the microalgae strains to fill those needs.

Analysis of the different microalgae did not find a clear pattern between specific nutrients and ability to support growth or development in early life for the clams, said the researchers.

The best preforming two-strain diet for larval clams had an EPA:DHA ratio of 2.43 and an n-3:n-6 ratio of 2.59, they said. The juvenile CM and TL diet had an EPA:DHA ratio of 1.84 and an n-3:n-6 ratio of 2.11.

“These fatty acids have been shown to be important in other bivalve juvenile species and so we expected those would be important,” ​said Pearce. “But there’s a lot of work in terms of what could be done [for] other components – certain proteins or carbohydrates.”​