Shellfish Industry Impacted by Ocean Acidification

Katie Haigh has a bachelor of science degree in molecular, cellular and developmental biology from the University of Washington and a master of science degree in environmental engineering from Washington State University. She has worked as a research technician at Taylor Shellfish since 2010, focusing on issues of water quality and shellfish nutrition.

Oyster Reproduction is in Decline

In 2005, for the first time on record, Pacific Oysters failed to reproduce naturally in Willapa Bay, a problem that has continued for seven years. To fill the gap created by this failed natural production, the shellfish industry turned to Northwest hatcheries to provide seed where oysters were otherwise disappearing. By 2006, the Taylor Shellfish Hatchery alone was producing 2.3 billion oyster larvae to supplement the industry’s growing needs, a labor-intensive but successful solution to the problem. In 2009 however, a second reproductive failure, this time at the hatchery level, further exacerbated the seed shortage. At Taylor Shellfish, despite doubling the hatchery’s production capacity, larvae production numbers had fallen to 1.6 billion — a 30 percent reduction from 2006. The Whiskey Creek Hatchery on the Oregon Coast saw production fall by 75 percent. These hatchery failures were a serious blow to the already struggling industry, spurring the growers and their academic allies to get a better handle on the water conditions required by oysters to survive.

Looking for the Culprit

It was first thought that the hatcheries were experiencing Vibrio tubiashii infections, a bacterium which had previously caused similar mortality at hatcheries. That year, however, the hatcheries were detecting neither the telltale signs of Vibrio infection nor the organism itself. Looking for other potential causes, their attention was drawn to a growing body of research on the topic of ocean pH declining due to ocean acidification, and the hatcheries began investigating water chemistry as a possible culprit in this growing problem.

Ocean Acidification Is a Problem for Shellfish

Concern over the issue of ocean acidification escalated rapidly, and in 2010 its possible involvement in Northwest oyster failures reached national recognition when U.S. Senator Maria Cantwell (D-Wash) helped secure funding for Northwest research institutes and shell fishermen alike to set up systems to study water quality. These systems have made it possible for shellfish hatcheries like Taylor Shellfish’s to monitor and adapt to varying seawater conditions.

Ocean acidification is the process by which oceans absorb excess atmospheric carbon dioxide, triggering a chain reaction culminating in a drop in ocean pH. Hard-shelled organisms like oysters and other shellfish rely on a mineral known as calcium carbonate to build their shells. As ocean pH declines, the availability of this mineral diminishes and it becomes increasingly difficult for these organisms to grow and thrive. A calcium carbonate deficiency is especially damaging during the first few days of the oyster life cycle. During this time, oysters utilize a carbonate species called aragonite, which is more highly affected by pH fluctuations than the species calcite, which is used later in the oysters’ adult life. When discussing carbonate availability, oceanographers use the term “saturation;” a saturation state of one is generally considered to be the threshold below which calcifying organisms like shellfish struggle to build shell.

A second phenomenon with the potential to magnify the problem of ocean acidification in the Pacific Northwest is “upwelling.” Upwelling is a regional occurrence by which wind currents on the water surface near shore cause deep ocean water to be brought to the surface. Deep ocean water is naturally characterized by higher carbon dioxide concentrations, lower pH and lower aragonite saturation. Many of the Northwest oyster beds and the hatcheries that supply them are located in areas susceptible to upwelling, and the damaging potential is enormous for these oysters if this deep water surfaces at the wrong time. Historically, however, upwelling itself has not been sufficient to cause the mass mortality that has been seen in the last few years. When paired with ocean acidification, which sequesters much of its absorbed carbon dioxide deep in the ocean, the waters brought to the ocean surface during these upwelling events will become more and more toxic.

What Does This Mean for Taylor Shellfish?

At the Taylor Shellfish Hatchery, the new focus on ocean acidification and the Cantwell funding meant an influx of water quality monitoring equipment and new research objectives.

The primary research goal was to correlate natural fluctuations in water chemistry to fluctuations in larval production success. The first part of this goal was met by the installation of an extensive water chemistry monitoring network to track water chemistry changes in and around the hatchery.

Between the 2010 and 2011 production seasons, five different probes were installed at fixed locations in and around the Taylor Shellfish Hatchery, to continuously monitor raw water conditions such as pH, temperature, salinity, dissolved oxygen concentration and oxidation reduction potential (ORP). They are still in operation today, and record data for both shallow and deep water sources. Shallow water conditions are measured five feet below the surface, and deep water at a depth of about one hundred feet. Deep water conditions are indicative of what might be seen by shellfish during upwelling events. A sixth mobile probe allows the same set of parameters to be measured at any location throughout the hatchery. A “pCO2 monitor” is the largest piece of equipment the hatchery has installed, and was acquired through a collaboration with Burke Hales at Oregon State University. This monitor continuously records carbon dioxide levels in the hatchery’s incoming water. Using these data, hatchery researchers are also able to calculate the aragonite saturation in hatchery waters.

Since 2012, a new generation of monitoring units was added to the scheme. An array of Durafet pH probes were installed to both monitor water conditions and control treatment systems. These are pieces of equipment whose precision had been oceanographer-approved, and allowed Taylor Shellfish to more accurately track the rise and fall of pH at their hatchery.

To achieve the second part of this objective, Taylor Hatchery researchers developed a system to determine larval success by characterizing larval performance in terms of growth and survival. For each of the season’s nearly sixty spawns, researchers generated growth curves based on microscopic measurements made every two days. These data allowed for an easy side-by-side comparison of each spawn group, and also allowed them to calculate an average daily growth, as measured in micrometers. Survival was also measured at three key stages throughout the production process. Their ultimate goal was to be able to correlate fluctuations in water chemistry to fluctuations in larval performance.

On top of working to correlate existing water conditions to real-life production numbers, Taylor Shellfish has also run production-scale experiments, manipulating the water conditions in which oyster larvae are spawned and raised. With declining ocean pH and aragonite saturation in mind, researchers have devoted a good deal of time to learning how larval oysters perform under conditions of varying pH and carbon dioxide. They have tested treatments to raise the pH, and thus the aragonite saturation of deep seawater, to try to determine if acidic waters can be made habitable, and they have increased the carbon dioxide levels in otherwise “good” waters to see how oysters will perform as the oceans continue to take up atmospheric carbon dioxide, or during times of upwelling.

How Have These Data Been Put to Use?

The Taylor Shellfish Hatchery had a record-breaking production year in 2010, with the hatchery generating over seven billion larval oysters, more than four times as many oysters as were produced the previous year. By the middle of the following summer, however, larval mortality again began to increase. Based on small-scale water treatment experiments that had begun the year before, hatchery workers moved to treat all of the facility’s incoming water with sodium carbonate (Na2CO3) in a partly-automated, partly-manual system that involved three separate but identical setups. Sodium carbonate is a salt that, when added to seawater, not only increases pH but also increases the availability of the larval oysters’ shell-building material, aragonite. The efficacy of the treatment system was reproduced on the large-scale, as it appeared to improve both larval growth and survival, and between 2011 and 2013, the water treatment system was tested, refined, scaled up, and made fully automatic. Today there is a single treatment system that operates around the clock at the Taylor Shellfish Hatchery. It is located in line with the hatchery’s main intake pipe and employs pH-sensors that control the injection of a sodium carbonate solution as needed to achieve and maintain optimal aragonite saturation.

What Does It Mean for the Future of the Hatchery?

The problems of declining ocean pH and aragonite saturation are ones that will not be resolved in the short term. Because of ocean circulation patterns, scientists estimate that the water being upwelled in the Pacific Northwest today was last at the ocean’s surface between 30 and 50 years ago. It follows that the carbon dioxide currently being absorbed in surface waters and sequestered in the ocean’s depths will not surface for another 30 to 50 years. As a result, fluctuating pH is an issue about which shellfish hatcheries like Taylor’s will have to remain vigilant, monitoring water quality on a continuous basis and being able to react to adverse changes on a moment’s notice. The hatcheries are lucky in this sense; because they are able to control their water source, they can head off problems related to water conditions. Unfortunately, calcifying organisms in the wild will not be so lucky.