In the 1981 blockbuster film “Raiders of the Lost Ark”the nefarious French archaeologist Belloq holds up a cheap pocket watch and explains to Indiana Jones “It’s worthless. Ten dollars from a vendor in the street.  But I take it, I bury it in the sand for a thousand years, it becomes priceless…

Here in the Delta, fisheries monitoring data are treated a lot like Belloq’s watch. An ancient, sandy watch isn’t valued for keeping precise time. Just the same, long-term fisheries monitoring data aren’t necessarily valued because they provide information effective for monitoring and managing Delta fishes. Consider one of the simplest fisheries monitoring tools; the seine, a length of net stretched between two poles. A seine hauled through shallow water provides a sample of the fish residing in that area. The scientific value of a single seine haul might be described as worthless.  However, the same seine pulled monthly for one hundred years would yield data that many fish biologists would consider priceless. 


Unfortunately, a well-intended but misplaced appreciation for the value of historical, long-term fisheries methods and data has been, and continues to be, an impediment to progress.  Long-term fisheries data, like Belloq’s watch, is a blunt tool for describing course changes over time, but the quaint appeal of history is not a substitute for monitoring designed to address contemporary management needs at shorter temporal scales. Meeting immediate information needs regarding juvenile salmonids entering the Delta requires a different, more tailored approach. 

Juvenile salmon and steelhead monitoring


The management and conservation of salmon and steelhead requires reliable estimates of freshwater juvenile production.  Juvenile salmonids enter the Delta from November through June at life stages ranging from fry (~40 mm in length) to smolts (~80 to 100 mm) or yearlings (> 125 mm).  Appropriately designed monitoring can provide juvenile abundance estimates necessary for assessing benefits of habitat enhancement actions, quantifying freshwater rearing capacity, evaluating effects of water project operations, and diagnosing problems in early development and survival.  Juvenile abundance estimates can also be used to forecast the abundance of adult salmon, thereby informing allowable harvest in recreational and commercial fisheries. In designing a monitoring program to estimate the abundance of juvenile salmonids, the key question is — what level of accuracy is needed to inform management decisions?


A reasonable goal would be an abundance estimate that is accurate to within 15% to 25% of the true abundance 90% of the time. Given our limited supply of water, the economic and societal value of salmon and steelhead, and the vast resources already devoted to studying, managing, and litigating these issues, one might fairly assume data at that level of resolution are being gathered. And one might also assume that if such data are not available, it must be because we lack the resources or know-how to obtain it. Although those are reasonable assumptions, both are incorrect.


How are we monitoring juvenile salmonids in the Delta?


Currently there are three predominant types of sampling relevant to juvenile salmonids in the Delta and its tributary rivers — trawling, seining, and rotary screw trapping. Like the sampling for delta smelt, most sampling efforts for juvenile salmonids use trawling. Trawls certainly capture juvenile salmonids but are problematic for estimating abundance or characterizing habitat use. Tagging studies have demonstrated low and highly variable capture efficiencies for trawls targeting juvenile salmon in the Delta. As with delta smelt, trawl-based juvenile salmon abundance estimates are problematic.  Over the last decade, several agency-funded efforts have attempted to derive juvenile salmonid abundance estimates from Delta trawls, but to-date none of these efforts have produced abundance estimates sufficiently accurate to inform management decisions. Despite these problems, the Delta Stewardship Council this year appears to have awarded funding to another effort to estimate juvenile salmon abundance from Delta trawls, this time specifically targeting spring-run Chinook.  


Seines are effective for capturing juvenile salmonids when the fish are holding or rearing along shallow channel margins.  But seines can only be deployed successfully where substrates are smooth and relatively uniform (for example, in sand) and where there is open space along the bank to pull the seine up and out of the water. Accordingly, seine-based monitoring in the Delta targets boat-ramps and other features that are not generally representative of salmon rearing habitats. Like trawling, agency-funded efforts have attempted to extract abundance estimates from seining, but those efforts have not been fruitful.   


Rotary screw traps (RSTs) are deployed in rivers — they do not function in the tidal Delta — to capture juvenile salmon and steelhead as they migrate downstream.  If properly placed and operated, RSTs can yield robust abundance estimates. Two RSTs are currently operated at Knights Landing; they are considered a primary source of data for juvenile salmonids approaching the Delta. Though those RSTs were ostensibly installed to provide information about how many juvenile salmonids are approaching the Delta, they are now recognized as limited to only providing an indication of when migration is occurring. The Knights Landing RSTs are not designed or operated to provide abundance estimates. Efficiency estimates for the Knights Landing RSTs are not available, and they are likely too low and variable to yield management-relevant abundance estimates.


How can we get the information we need on juvenile salmonids?


Reliably estimating the abundance of juvenile salmonids approaching the Delta (or in upstream tributaries) requires a monitoring program that satisfies at least three criteria. First is a rigorous sampling design that is unbiased and randomizes days (or hours) sampled out of the available sampling period. Second is to assure that the capture efficiency is greater than one percent, recognizing that monitoring methods with lower capture efficiencies are prone to overdispersion – that is, allow excess noise in the data — and resulting abundance estimates can be unacceptably inaccurate. Note that releasing acoustically tagged fish can yield good point estimates of capture efficiency for juvenile salmonids large enough to be tagged, but there is no evidence to suggest that this approach will allow trapping programs with inherently low and highly variable capture efficiencies to provide accurate abundance estimates. Third, rigorous capture efficiency experiments and modelling need to be carried out. Capture efficiency can be strongly influenced by species, year, life stage and size, as well as river discharge and turbidity. Conducting a handful of efficiency experiments each year with fall-run Chinook fry, the most common species and life stage, is an inadequate basis for estimating abundance of other runs and species and life stages. Trap efficiency experiments need to utilize large release groups (more than 500 individuals) and target species, life stages, and environmental conditions known to affect capture efficiency.  


While these criteria for reliably estimating outmigrant abundance are relatively well understood in the scientific literature and among experts, few monitoring programs operating in Central Valley tributaries satisfy them. The Red Bluff Diversion Dam (RBDD) winter-run Chinook trapping program is one of the most robust of the nearly 20 Central Valley RST programs. While the RBDD program achieves consistently good performance in capture efficiency for fry-sized fish, its capture-efficiency model does not account for effects of fish size, species, or turbidity. Furthermore, the RBDD does not always follow a randomized sampling design; sampling often stops for consecutive days during periods of elevated flows with debris or during releases of hatchery fish.


The best approach to estimate abundance of juvenile salmon entering the Delta with necessary rigor is to implement trapping programs consistent with the criteria described above. One trapping program to estimate production originating from the Sacramento River basin, and another for production originating from the San Joaquin River basin.  


What are the obstacles to getting reliable juvenile salmonid production estimates?


Clearly defining the accuracy and precision of data needed to satisfy the information needs of resource managers is the critical first step in designing an effective monitoring scheme for juvenile salmonids.  Tools like statistical simulation can then be used to provide managers with an opportunity to understand the value of improved monitoring before it is even deployed.  Lacking clear objectives, those charged with improving monitoring often reframe the challenge as “How can we leverage existing surveys to get more and better information from them?” Although it is useful to improve data from existing surveys, there is no substitute for carefully defining what we need to know and openly considering what methods and sampling design are necessary to get that knowledge. 


Another obstacle to meeting contemporary information needs is that fish biologists tend to favor familiar methods and often eschew solutions that involve new technology or engineering. As noted above, achieving higher capture efficiencies and the ability to operate safely during elevated flows are major impediments to effectively estimating juvenile production with RSTs. Engineering-based improvements like pilings, debris booms, fish behavioral barriers, and floating work platforms would provide substantial benefits to juvenile salmonid monitoring efforts. Yet these improvements seem to be outside the scope of what most fish biologists consider feasible, or beyond what managers are willing to pay for.


And then there is permitting. Capturing juvenile salmonids requires at a minimum, a scientific collection permit, but compliance with federal and/or state Endangered Species Acts is almost always necessary. Though regulatory agencies strongly support getting estimates of juvenile salmonid abundance, there is an inexplicable cognitive dissonance between that desire and the quantity of take that the agencies typically allocate. As described above, catching at least one percent of juveniles is a pre-requisite for meaningfully estimating salmonid abundance.  Yet permitted take for ESA-listed species is often much less.  Many existing RST programs operating in the Central Valley routinely curtail their sampling to avoid exceeding a take limit that was set arbitrarily – without the benefit of knowing how many fish are present or how many fish must be sampled to estimate abundance.


Accurately estimating the abundance of juvenile salmonids entering the Delta will never be easy, but it is an achievable goal that is long overdue. While funding for biological monitoring is often limited, accurately estimating juvenile salmonid abundance would provide a high return on investment for managers of fish and water.  If cost proves to be an obstacle, retiring ineffective juvenile salmonid monitoring programs could help make up the difference.  Belloq’s watch has its appeal, but at this point we need to consider trading it in for a more functional timepiece.  

No Comments
Comments to: How many juvenile salmonids reach the Delta each year? We don’t know.

Your email address will not be published. Required fields are marked *

Attach images - Only PNG, JPG, JPEG and GIF are supported.

Thank you! Your subscription has been confirmed. You'll hear from us soon.
Subscribe to DeltaCurrents