Rockfish Report

Trends in Abundance and Management of Striped Bass Along the Atlantic Coast: Can We Have too Much of a Good Thing?
September 2014 Desmond M. Kahn, Ph. D.

In response to a decline in abundance of striped bass over the last decade, some recreational fishers on the Atlantic coast have been calling for reductions in striped bass landings by both recreational and commercial fisheries. In fact, some recreational fishers have been opposed to commercial landings of striped bass for some time, and instead would prefer to have the species classified as a game fish which cannot be sold.

Striped bass are one of our great inshore fishery natural resources along the Mid-Atlantic and southern New England coastline. If commercial landings were banned, then people without the time or money to fish would not be able to eat striped bass, known locally as rockfish. It is a traditional delicacy, at least in Delaware and Maryland. Consumption of fish such as striped bass has great benefit to human health. We would then have to import even more of our seafood from Asia than we already do, to replace that lost source of fish in our diet, consequently worsening our trade deficit. Then what would all the people who now earn some of their living from the commercial striped bass fishery do for income, including not just fishermen, but netmakers, boat builders, people who work at seafood dealers and truck drivers who haul catches? Seen in this light, restricting the landing of striped bass to recreational fishers has severe drawbacks from the point of view of society as a whole.

The Atlantic States Marine Fisheries Commission, which manages striped bass, has begun formal consideration of some form of reduction in allowable landings, by creating a Draft Addendum to the current Fishery Management Plan. Such draft addendums are usually the subject of public hearings held in various locations along the coast to obtain public feedback.

There is no question that bass have declined in abundance from their peak in the early 2000s. Figure 1 shows the trend in abundance of striped bass between 1981 and 2012. This trend is based on an index of changes in the total number caught per recreational fishing trip, including bass released, in Mid-Atlantic waters. Bass abundance was very low in the 1980s. By the early 1990s, though, abundance began to increase steadily, reaching a peak in the early 2000s. Since then, abundance declined to roughly the level of the mid-1990s. Although this chart through 2012 does not show it, in 2013 total abundance measured by this index has begun to climb as the fish born in 2011 have been entering the fishery.

Note, though, that the index in Figure 1 reflects the total number of fish. Another measure of stock size that the current Management Plan places great emphasis on is the total weight of mature female bass in the stock, known as female spawning stock biomass. This quantity can only be estimated by mathematical population models, and these models are subject to bias and errors. Since female bass do not become mature currently until age seven or eight, the total abundance can be out of sync with the total weight of mature females. For example, it will take roughly seven years until the fish born in 2011 will begin to influence female spawning stock biomass.

Some people have raised fears that the recent decline could be the beginning of a drop similar to the very low abundance we saw in the 1980s. There are a couple of reasons why that is unlikely. One reason is that we now have a restored Delaware River spawning stock of striped bass, which was barely beginning to return to life during the 1980s, having been almost extinct for decades due to lack of oxygen in the tidal Delaware from Philadelphia through Wilmington. This stock was formally declared restored in 1998, and produces roughly 10% to 15% or so of the total Atlantic stock. Beginning in the 1970s and continuing through 1987, funding provided by the federal Clean Water Act allowed upgrading of sewage treatment plants. Industrial discharges were also cleaned up, oxygen levels increased and bass were again able to spawn successfully from Wilmington through Philadelphia and survive and grow.

The second reason is that the fishery regulations formulated in the Commission are now mandatory for all states, due to a federal law passed since the low point of striped bass abundance in the 1980s. These regulations impose a conservative creel limit of two fish of either 18 inches in some estuaries or 28 inches elsewhere. Commercial landings are restricted to quotas which generally are constant for at least a number of years.

Then, you may ask, why has bass abundance declined? That is a good question and we have great difficulty proving that one reason or another was the cause. As the science of biology has shown, fluctuation in abundance of wild animals and plants is the rule, not the exception. It seems clear, however, that the decline in coast-wide abundance occurred because no dominant year-class was produced in the Chesapeake Bay for seven years between 2003 and 2011. Figure 2 shows the Maryland Fisheries Administration’s striped bass young-of-year index, based on the mean catch per haul of a beach seine at selected locations in the Maryland waters of the Chesapeake Bay. The Bay produces roughly 70% of bass on the coast, with the Hudson and the Delaware combining for roughly the other 30%. The Bay stock has a pattern of producing abundant young bass at irregular intervals, known as a dominant year-class. The fish born, or produced, in a given year are known as a year-class in fishery terminology. These dominant year-classes tend to support fisheries for a number of years, particularly in the past when more liberal regulations would allow heavier fishery landings than are allowed today.

The lack of a dominant year-class for seven years meant that the number of young fish issuing from the Chesapeake to replenish the coastal complex declined. This reduction of younger fish in the pipeline caused a gradual reduction in total abundance as the older fish, many from former dominant year-classes, gradually died off. The 2011 year-class reversed this trend, and as these fish begin to migrate along the coast, anglers will experience higher abundance of smaller, sub-legal fish initially, just as we who fish in the Bay itself have experienced very high catches of small fish in the last couple of years.

But what caused the gap in production of dominant year-classes? That is a more difficult question. Current fishery management aims to protect adequate spawners to produce enough eggs (and hopefully, sperm) to replace fish that die. The Chesapeake Bay has not had any lack of female spawners during 2004-2010. Under the current conservative regulations, we have seen large numbers of bigger old fish, meaning that the survival rate is high enough that females, in particular, can live to older ages. Research conducted on the Delaware River spawning stock, for example, has demonstrated that both the mean age and the mean length of females collected by electrofishing on the spawning grounds have shown a statistically significant positive trend over the period of 1994 through 2010.

In the Chesapeake Bay, an outstanding dataset of striped bass egg abundance has been developed by Maryland biologist Jim Uphoff, who you may have read about in Dick Russell’s book, “Striper Wars”. This data stretches all the way back to the 1950s, which is quite rare in fisheries data, and shows clearly that, while egg abundance declined noticeably in the 1980s, there has been no decline in recent years.

On the other hand, several peer-reviewed scientific papers have documented negative effects of high density on the resident fish in the Bay. These are largely males, since the females tend to migrate along the coast. This research has documented starvation, reduced forage abundance, the spread of a wasting disease similar to tuberculosis produced by Mycobacteriosis bacteria, and increased mortality among the residents that has not been due to fishing. One hypothesis that these results suggest is that there could have been a negative impact on male fertility as a result of the high density. Since striped bass, like most fish, are broadcast spawners, a high ratio of males to females in the act of spawning is required to achieve high fertilization rates. Another hypothesis is that, despite adequate numbers of both female and male spawners, a chance string of unfavorable conditions has worked against production of a dominant year class during the years from 2004 through 2010.

Turning to the science of biology again, its branch of ecology has found that high densities of wild animals or plants tend to develop negative feedback on a population’s growth rate, causing reductions in survival, disease outbreaks, reduced growth of individuals and a decline in the reproductive rate. This general pattern is consistent with the current state of striped bass, at least for the resident fish in Chesapeake Bay. The Bay is so long that it bears some resemblance to an enclosed lake when compared to Delaware Bay or Long Island Sound.

If we want a balanced set of fisheries on the Atlantic Coast, we should seriously consider managing for bass abundance similar to current levels, that is, below the peak levels of the early 2000s. I say that because biologists have developed evidence that the peak abundance of striped bass produced noticeable reductions in abundance of some other species and negative impacts on their fisheries. This includes weakfish, American shad, river herring (blueback and alewife) and menhaden or bunker. Stripers are piscivores, meaning they prey on other fish. Ecology has found that when a predator increases in abundance, its prey tends to decrease in abundance. However, the Commission has so far remained stuck in the “single-species” mode of management that ignores interactions between species, going so far as to suppress scientific findings implicating striped bass in declines of shad and herring in the Connecticut River.

The Commission’s coastwide stock assessment of weakfish, which I worked on extensively along with biologists from Rhode Island to Florida, found in 2006 and again in 2009 that that species had declined significantly, but that the decline was not due to fishing, which had been regulated in the 1990s. Instead, our assessments found that the weakfish decline was caused by predation and possible competition from striped bass and spiny dogfish. The return of recreational catches of weakfish along the Delaware coast as bass abundance has declined in the last couple of years is consistent with that finding finding as bass abundance has dropped back down towards the level seen in 1995.

Scientific dietary studies of striped bass have found that their preferred prey consists of members of the herring family, including shad and menhaden, or bunker. American shad abundance declined in rivers coastwide during the late 1990s and the 2000s. Managers blamed this on the coastal gill net fishery for shad and shut it down by 2005. If that fishery had been the cause of the shad decline, they would have bounced back immediately. However, the runs failed to improve. A couple of years ago, while working with biologists from the four states in the Delaware River Basin on a stock assessment of the Delaware River American shad spawning stock, I plotted abundance trends of that run with an index of abundance of striped bass in the state waters of Delaware (Figure 3).

The result amazed us. There was a very highly significant negative correlation between the two. Figure 3 shows that shad in the Delaware were at their peak when stripers were at their lowest abundance in the 1980s. As bass rebounded, shad declined. Although that graph only extends to 2010, as the bass decline continued, the Delaware shad runs have rebounded strongly in the last few years. This pattern is consistent with the hypothesis that striped bass suppress American shad abundance through predation. Victor Crecco and Tom Savoy of the Connecticut Bureau of Marine Fisheries have published papers documenting a similar pattern in the Connecticut River. Field work by University of Connecticut student Justin Davis documented that large bass in the River ate adult American shad.

River herring are a prime prey for bass in spring as both run up freshwater streams to spawn. Environmental groups have tried to implicate commercial fishery bycatch as the cause of the reduced abundance of herring, but as bass have declined, river herring have rebounded in recent years. The above researchers on the Connecticut River documented major predation by bass in spring as the cause for a decline in herring abundance in that River.

In conclusion, striped bass are experiencing some normal fluctuations in abundance, caused by a gap in production of dominant year-classes from the Chesapeake Bay. The high abundance of resident bass in the Bay has had a negative feedback on that stock, and could possibly even have cause reduced fertility of the Chesapeake spawners. The 2011 year-class has reversed a declining abundance coast-wide and, in any case, the current lower coastwide abundance of bass has allowed a rebound in abundance of weakfish, shad and river herring. This recent lower level of bass abundance has restored some balance to our coastal fisheries and could be a more desirable target for management than the higher bass abundance we experienced in the early 2000s.

Figure 1. Trend in abundance of striped bass in the Mid-Atlantic region (raw data on fishing trips and total catch obtained from NOAA Fisheries.)

Figure 2. Average number of young-of-year striped bass collected per haul of a beach seine by the Maryland Fisheries Administration from 1957 through 2012.

Figure 3. Relative abundance trends of American shad in the Delaware River and striped bass in waters of the state of Delaware from 1981 through 2010. Courtesy of the Pennsylvania Fish and Boat Commission and NOAA Fisheries.

John Clark holds a female weighing approximately sixty pounds collected on the Delaware River spawning grounds. Courtesy of the Delaware Division of Fish and Wildlife.

• This topic was modified 1 year, 2 months ago by  popper.