In aquaculture, the effect of temperature changes on the feeding status of oysters
preface
Europe produces two types of mussels and two types of oysters, with aquaculture producing the highest blue mussel (Ranunculus) at 130 000 tonnes in 2018, followed by Mediterranean mussels (Pityriabacterium gallicus) and Pacific oysters with about 100,000 tonnes and European flat oysters (Ostrich) with 2,500 tonnes.
Currently, seafood accounts for only 17% of edible meat, and by 2050, an increase of 36-74% of marine protein is needed to meet the world's nutritional needs, and bivalves are one of the most sustainable foods of animal origin, underutilized, considered part of the food of the future, and meet many of the United Nations Sustainable Development Goals.
1. Bivalve shellfish culture under natural conditions
Increasing production under climatic stress can be a challenge because of the strong interaction of bivalves with the environment, which is currently dominated by natural supplementation, which in turn is influenced by environmental factors such as food availability and water temperature salinity.
Bivalve shellfish do not feed on culture, but naturally feed, directly from the water column, do not need to feed, so production depends on environmental conditions, the main physical factors affecting their distribution are temperature, affecting the survival and growth of adults and larvae, bivalves are sensitive to temperature and salinity changes caused by climate change, these changes affect behavior, physiological rate and immune system.
Second, the impact of temperature changes on European oyster farming
Recently, FAO summarized the available evidence on the impacts of climate change on aquaculture, concluding that the short-term impacts of climate change on aquaculture may include loss of production and infrastructure, disease, parasites and increased risk of harmful algal blooms due to extreme events such as floods.
Long-term impacts may include reduced availability of wild seeds as well as reduced precipitation, increased competition for freshwater, climate-driven changes in temperature, precipitation, ocean acidification, the incidence and extent of hypoxia, and sea level rise.
Multi-scale long-term impacts are expected on the aquaculture sector, with stressors associated with climate change, such as increased average temperatures, acidification and hypoxia, affecting growth, and extreme temperatures affecting survival, which in turn affects productivity.
Third, the effect of food concentration on the growth status of two oysters
The survival rate of bivalves during the experiment ranged from 93% to 100%, except for mussels exposed to 30°C, all of which died during these treatments.
After three to five days, the time of buttercup and Mycobacterium galloprovasia was significantly longer, with low food concentrations for 16 days and high food concentrations for 32 days, survival rate was 99.46%, only 12 out of 2214 mussels died in the experiment, and the mortality rate did not follow a specific treatment pattern.
A significant effect of food on growth was observed, with a maximum shell growth rate of 0.0283±0.0060 mmd−1 at low food concentrations, compared to 0.1703 ± 0.0297 mmday−1 at high food concentrations.
The maximum specific growth rate of mussels was 0.2651±0.0191 at low food concentrations and 1.7697±0.2177 at high food concentrations, and the maximum shell growth rate of oysters was 0.0616±0.0115 mmday−1.
The results showed that at low food concentrations, while 0.2548 ± 0.0279 mmday−1 at high food concentrations, the maximum specific growth rate of oysters at low food concentrations was 0.5608±0.0856 and at high food concentrations 2.2855±0.2169, and the growth results showed that the effect of temperature was influenced by significant interactions with food, with higher food conditions showing higher optimal temperatures for mussels and oysters to grow.
The condition of shellfish is affected by temperature and food and their interactions, the higher the food concentration, the higher the condition index, which decreases with increasing temperature, in addition, the condition index of Pacific oysters is significantly higher than that of flat oysters.
Fourth, which of the two oysters will have a higher survival rate at different temperatures?
Experiments showed that mussels can survive in the temperature range of 3-25 °C, but not at 30 °C, the experimental results showed a sharp increase in mortality at all temperatures above 30 °C, Mesas and Tarifenio, and also determined an upper lethal temperature of 30 °C, Mycobacterium galloprovonia, showing survival rates of more than 9 weeks for Mycobacterium galloprovonia.
Juveniles from hatcheries in the United States were exposed to 30 °C and had higher survival rates under high food conditions, however, Mycobacterium galloprovin juveniles confirmed these findings but showed a shorter survival time of 4 weeks, experiments showed that both oysters survived at 30 °C, O. edulis has been shown to survive at 30 °C, but not at 36 °C.
Fifth, the main reason for the decline in the growth rate of mussels and flat oysters
The growth rate and condition index were higher at higher food concentrations, and higher food conditions showed higher optimal temperatures for mussels and oysters to grow, with an optimal temperature of 8°C for low food conditions and 15°C for high food conditions.
Pacific oysters grew best at 15 °C under low feeding conditions and 20 °C under high feeding conditions, while flat oysters grew best with low feeding at 20 °C and high feeding at 25 °C, and there were few studies on the interaction between these two factors.
As the food supply increases, one potential explanation for the shift to higher optimal growing temperatures could be that bivalves can better cope with higher temperatures when more food is provided.
All bivalves, except Mycobacterium galloprovin showed some growth at 3°C, however, the long-term effects of low temperatures on oyster growth and condition are unknown, with juvenile weight loss observed at 3°C during an 11-week experiment.
Author's point of view
In addition to the environmental impact of climate change, other factors such as invasive species, farming practices and diseases can affect bivalve productivity, modeled using an integrated approach, including physiological responses to multiple environmental factors that are relevant to managers and industry.
