Science Advisory Report  2013/062

Identification and evaluation of biological effects and impacts of sediment to sponge communities in Hecate Strait

Summary

• Glass sponges are animals with tissues and sensory systems, but they are non-motile due to the absence of any muscle. Therefore, they are dependent on underwater currents for delivery of food. The few places where glass sponges occur in high densities exhibit a balance between high turbidity (natural fallout that delivers nutrients) and sufficient current to prevent accumulation of sediment and smothering.
• Sponges feed by filtering water for digestible organic material, largely bacteria.  Exposure to suspended sediment (above background levels) causes arrest of the feeding current through the sponge. Prolonged exposure (>40min) causes clogging of the sponge filter and a reduction in the volume of water processed and in nutrient uptake. For reef building glass sponges, loss of energy supplies during summer periods of peak feeding could compromise growth and future reproductive ability.
• Indirect effects due to resuspension of sediment from human activities have been documented to affect sponge communities including hexactinellid ‘glass’ sponges. Fishing activities that re-suspend sediment, due to contact with the bottom, have historically occurred, and continue to occur, in the Adaptive Management Zone (AMZ) of the proposed Hecate Strait and Queen Charlotte Sound Area of Interest (AOI).
• This assessment did not consider the risk to the ecosystem from impacts to sponge reefs. In the absence of a detailed understanding of the ecosystem consequences of exposure to sponge reef communities, a precautionary approach is recommended considering the mitigation scenarios that reduce the risk of exposure.
• An assessment was conducted of the risks to sponge reefs associated with remobilization of sediment from fishing activities. The assessment considered both  the physiological consequences of sediment on glass sponges and sponge reefs and the level of exposure or the impact of remobilized sediment from various types of fishing, which have historically occurred in the Adaptive Management Zone  (AMZ) of the Hecate Strait and Queen Charlotte Sound Sponge Reefs.
• The assessment found that the risk associated with fixed and mobile fishing gear caused by remobilization of sediment is “high”, and may cause deleterious effects on sponge reef organisms.  As a result, a higher level of scrutiny and potential management action are recommended.
• A number of factors related to the potential level of exposure were explored quantitatively. Factors that affect the rate and range of exposure of sponge reefs to re-suspended sediment were identified to include: the interaction of fishing gear with the bottom, which determines the height to which sediment is ejected in the water column; sediment composition, which determines the length of time sediment is suspended in the water column; and, current speed, which determines the distance and direction over which sediment is carried in relation to the location of the fishing event.
• Exposure was assessed using data and models to understand: the level of interaction of fishing activities with benthic sediments; the historic locations and timing of fishery activities in the AMZ; the sediment types surrounding each reef complex; and, the factors that affect the resettlement rates of the remobilized sediment.
• The findings of the sediment dispersion models were used to identify the risk of exposure to the sponge reef from remobilized sediment under six mitigation scenarios, and are presented for management consideration.

This Science Advisory Report is from the Identification and Evaluation of Biological Effects and Impacts of Sediment to Sponge Communities in Hecate Strait, Canadian Science Advisory Secretariat, Regional Peer Review meeting held October 23 to 25th, 2012. Additional publications from this meeting will be posted Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

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