Background. American shad were introduced to West Coast rivers in the late 1800s and rapidly expanded. Shad colonized upriver reaches of the Columbia River Basin as the hydropower system developed, using passage facilities provided for native salmonid species. The non-native shad is now the predominant anadromous fish species in the Columbia River Basin. Since the 1980s, shad routinely surpassed the combined total of all returning native salmon and steelhead adults counted at Bonneville Dam, while in some recent years they comprised greater than 90% of the recorded upstream migrants. In 2019, for example, more than seven million shad passed above Bonneville Dam by late August, compared with fewer than 250,000 adult wild anadromous salmon and steelhead and 450,000 hatchery salmonids. The opposing general trends for shad (increasing) and salmonids (decreasing) point to an ongoing, and perhaps accelerating, disruption of ecosystem health in the basin. Furthermore, the growth of shad populations risks straining the management and operational infrastructure aimed at stabilizing or recovering anadromous salmonid communities.
Problem Statement. At such high abundances, shad interfere with efforts to aid salmonid upriver adult returns by obstructing efficient processing of salmon at collection facilities, lowering oxygen concentrations in fish ladders, and complicating accurate identification of migrating salmon and steelhead in fish counting facilities. Additionally, the increasing trend in shad abundance has led many to speculate that they are an ecological hazard to native fish populations, including salmonids, during their downriver migration and estuarine or ocean residency. Potential hazards of increasing shad populations to native fishes include greater competition for food and critical nursery habitat, predation on salmon young, disease transmission, or some combination of these processes. Impacts of shad potentially compound the well-established effects of the “4 Hs” – hydrosystem, habitat, hatcheries, and harvest – on native fishes.
Despite their abundance in the Columbia River Basin, management agencies have displayed a limited interest in targeting management or sponsoring research programs focusing on shad, in large measure because of other critical priorities. However, concern over the shad’s impact is not new. In the 1990s when shad abundances were a fraction of their current levels, state, Tribal, and federal authorities called for exploring more active management approaches such as targeted suppression as a means to control potential interactions between shad and salmonids. Although the justification for such calls-to-action were largely precautionary given the paucity of directed data collection and analyses, neither a concerted suppression effort nor additional data collection and analysis were undertaken to any satisfactory extent.Consequently, much of the speculation surrounding shad’s biology (based on experiences and knowledge within their native range) and the associated impacts to the Columbia ecosystem remain as potential ecological concerns. Additionally, possible management actions to address the impacts are largely unexplored. Furthermore, the value of shad to the region and in the basin, whether cultural, commercial, or recreational, does not begin to compare with the iconic stature of native salmon and steelhead. As a result, the Independent Science Advisory Board (ISAB) was asked to examine the state of the science on shad’s impact and report back on what is confidently known about their role and impact in the ecosystem, about the areas needing additional investigation, and ultimately about how fisheries co-managers and hydrosystem operators might consider shad within the context of their activities.
General Approach. To complete this report, the ISAB reviewed published literature, reports, and other documents for any scientific information on shad in the basin. We conducted new analyses on the influence of ocean conditions on shad, population dynamics of shad in the Columbia, and the influence of temperature and discharge on timing of adult shad migration. We also received briefings from a Tribal fisher and a Tribal fishery program officer to get personal experiences and perspectives on shad. Last, we requested perspective from basin staff from Washington and Oregon state fishery agencies.
Highlights of the Report’s Findings. The full report addresses numerous topics for which we were able to credibly cite documents or sources. Here, we highlight a few key findings.
- The ISAB confirmed the upward trajectory of adult shad passing the lower dams over the past 60 years or so. Prior to 1960 adult shad were estimated to be fewer than 20,000 adults per year at Bonneville Dam. Their numbers rose to over 1 million per year after The Dalles Dam was completed in 1957, which flooded Celilo Falls, previously a barrier to the upstream migration of shad, apparently facilitating upstream access. Since 1960, shad have increased at an average rate of about 5% per year with no evidence to suggest this increase is slowing or leveling off. This trend is in direct opposition to what is occurring on the East Coast in the shad’s native range.
- Similar to their East Coast ancestors, shad migrate upriver and spawn in late spring through early summer, keyed by rising water temperatures and river flows. Typically, after hatching, young remain in-river for up to a year feeding on plankton but often migrate to the estuary or ocean in the fall to continue feeding and growing in the shallow coastal waters.
- Shad display a considerable variation in life-history, including the emergence of a unique form called “mini-shad” in the Columbia River.
- Little information is available about the shad’s coastal life experiences and diet except from incidental capture from other fishery monitoring efforts.
- Warming conditions in ocean, estuary, and river habitats predicted by climate change models for the Pacific Coast will probably favor shad because of their tolerances for warmer water than salmonids. As warming continues, so too will shad’s upward trend likely continue.
- Shad appear to display a relatively high fidelity to their natal rivers. While observed occasionally in rivers or offshore from Central California around the Pacific Rim to Russia, the populations associated with the Sacramento and Columbia Rivers are the largest.
Reasons for Shad’s Success in the Columbia. The success of shad introductions and the rapid expansion of the shad’s range along the Pacific Coast can be explained by understanding some basic facts about their biology:
- The flexible physiology of shad enabled rapid acclimation to the conditions of West Coast rivers. Shad tolerate a wide range of temperature (albeit warmer than salmon), flow, turbidity, salinity, and other aspects of water quality and quantity.
- Their anadromy and their low-level propensity to stray among rivers allow them to move from one basin to another, up and down the coast.
- Shad do not build protective redds, but rather broadcast their gametes. Developing eggs and hatchlings passively float downriver along with currents. They also engage in serial spawning in which gamete releases occur in pulses, such that their life-time fecundity is higher than salmonids.
- Their dispersal seems to be facilitated by the often favorable, if complex, set of ocean conditions.
- The series of hydropower dams and reservoirs in the Columbia River system has created favorable conditions for shad passage, spawning, and rearing.
Ultimately, the interactions between shad, salmon, and other parts of the Columbia River ecosystem will continue changing as temperatures increase, river flow regimes change, and sea-levels rise; changes that will likely lead to further native species decline while favoring some non-native species, like shad. The chain of reservoirs in the Columbia River will likely continue to provide near-optimal habitat for shad spawning and rearing. A warmer climate will likely favor shad over salmonids, although much depends on the continued productivity of the river and reservoirs, the estuary, and, above all, the ocean. The winds and currents that create this productivity may become even more erratic, so populations of shad, other fishes, and their predators are likely to fluctuate widely and unpredictably.
Ongoing and Future Risks to Columbia from Shad. We generally lack basic information on the many potential ecological interactions between shad and native species. Ultimately, the extent and impact – on balance – of any interactions have yet to be established with a robust analysis of empirical data. The very large numbers of non-native shad suggest long-term negative effects, but the nature of those effects, if indeed they are present, remains to be uncovered. If the trend continues, there is an emerging risk that the major anadromous fish production for the Columbia River Basin will be for a non-native species rather than salmon and steelhead.
Management Opportunities and Options. Shad were introduced to the West Coast because of their enormous popularity as a food and sport fish on the East Coast. On the West Coast today, they are no longer preferred by consumers to any significant extent given the wide availability and variety of other food fish. As a result, the small commercial fishery that existed in the river in the 1970s has given way to a sport fishery with limited closures and unlimited creel. Among the many reasons a more robust commercial fishery has not developed in the basin is that gillnets and other presently used commercial gear also capture endangered salmonids as unintended by-catch. Alternative selective gear types with low impacts on native fish are needed and are being explored.
Overall, there seems to be little interest or demand for Tribal shad fisheries, despite shad’s abundance. As a non-native species, shad do not have an apparent cultural role among the Tribes within the basin and will likely remain so into the future. To date, a formal presentation of Tribal perspectives on shad has not been released, except in the unstated context of salmon policies. The multiple Tribal perspectives are centrally important in assessing ecological and social impacts or benefits of shad populations in the Columbia River. These perspectives also are essential for assessing the current or potential commercial fisheries as a potential suppression strategy for shad.
In addition to changes in harvest, hydrosystem operations may have management opportunities. Sub-surface rather than surface entryways to fish ladders may ultimately serve to slow or reverse the population trends for shad.
Recommendations. Given the current state of understanding about shad in the Columbia River Basin, a number of critical uncertainties for adaptive management could be addressed by a more focused shad research and monitoring program than what is presently occurring.
Such a program might benefit from a formal scoping process among co-managers to identify and prioritize the critical uncertainties to be addressed, recognizing the merits and explicit costs of competing research priorities in the basin. In the interim, we recommend starting with a short-term goal of describing fundamental life-history patterns of the species in the Columbia River system: age-structure, habitat utilization, survival rates, primary predators, and prey. Critical uncertainties about conditions that might limit shad reproduction and survival without impacting native species is a key area on which to focus. At a minimum, ongoing monitoring of shad numbers passing the dams in the Columbia River and major tributaries is essential.
Prior to developing extensive field studies, modeling might be a prudent and cost efficient first phase of analysis. Such modeling could provide the basis to evaluate the potential importance of interactions between shad and anadromous salmonids and other native species. Specifically, the models might address whether shad either increase predation on salmon or act as a buffer that reduces predation. Another promising approach is bioenergetic modeling. Manipulation of the abundances of shad and salmon, and the degree to which they interact, in model simulations could initially assess which interactions may be ecologically and economically important.
Climate models predict that the basin and near ocean will experience climatic warming, hydrologic changes to the basin will continue, and increasing shad abundances show no indication of having reached a plateau. These emerging issues and potential risks warrant increased attention by resource managers in the Columbia River Basin to address uncertainties about possible shad effects on declining native species and to cultural practices.