Harvesting and the Spherical Cow: From Zebrafish to Menhaden to Red Snapper, by Don Orth

Regulating harvest is perhaps the most fundamental role for fisheries management.  The theory is far easier than the practice.  Developers of early fisheries theory (Beverton and Holt 1957) invoked the “spherical cow” in fisheries.  The spherical cow is a metaphor for mathematically describing the problem in the simplest form possible to make calculations more feasible. As we see with stories about the Zebrafish (Danio rerio), Atlantic Menhaden (Brevoortia tyrannus), and Red Snapper (Lutjanus campechanus), the simplification hinders application.  

Zebrafish female top, male bottom.  Photo by

Tohru Murakami

Zebrafish is member of the minnow family (Cyprinidae); it is native to the southeastern Himalayan region.  Zebrafish readily adapt to life in aquaria and they have a fast development time and short generation time; for these reasons they have become a model fish for laboratory investigations, typically development, gene function, and toxicology.

Atlantic Menhaden is a member of the herring family (Clupeidae); it lives in estuaries and coastal waters from Nova Scotia to northern Florida.  They are filter feeders that swim with their mouth open and gill openings spread.  They travel in large schools and are preyed upon by many fish, marine mammals, and birds,  Atlantic Menhaden mature in two years at a size of 18-22 cm; females produce between 100,000 to 600,000 eggs, depending on size.  Common names include the pogy, mossbunker, bunker, fat-back, and bug-mouth.  Native Americans called them 'munnawhatteaug' (=fertilizer).

 

Atlantic Menhaden

Red Snapper is a long-lived, early-maturing fish in the family Lutjanidae.  They are broadly distributed in the Gulf of Mexico and are an important sport and commercial fish.  Red Snapper have been overfished for several decades and are one of the most controversial fisheries with high landings from both recreational and commercial fishers.

Red Snapper caught in January 2013 was 38.25 inches and was released due to closed season.

Any valuable fish will be intensely targeted by both commercial and recreational anglers.   Intense fishing pressure will remove the large and mature members of the populations.  Consequently, truncated age distributions and artificial selection are pervasive fisheries problems.  The first response of managers is to enact a minimum length for possession that allows at least one successful reproduction before first harvest.  Yet this management approach facilitates higher harvest on larger fish, often leading to recruitment overfishing and lack of large fish.   Instead of more and bigger fish, the fishing public is left with fewer and smaller fish.  

This gradual decline in the average size of fishes caught in intensely harvested populations has been documented in numerous fisheries (Silliman 1975; Ricker 1981; Sharpe and Hendry 2009; Arlinghaus et al. 2010).  Cautious managers warned that intensive size selective fishing could lower fisheries productivity over time (Hutchings 2009; Rijnsdorp et al. 2010; Heino et al. 2013).  The effects of size selective fishing are not only plausible, but potentially widespread.   

   

The mathematics of optimum size based harvesting (Reed 1980) is based on the assumptions of stationarity and identical fish in each age group.  The population is further assumed to interact with no others (as predator or prey or competitor).  The maximization of yields based on Reed’s mathematical model demonstrated that under intense levels of exploitation, harvesting restricted to two age groups maximizes yields.  The mathematical theorem does not support the application of minimum size limits as an optimal strategy.  Rather, the optimal size to harvest depends on exploitation rate.   And old, more fecund age groups, are protected from harvest.

Annual steady state yield expected at different levels of exploitation.  The numbers on line segments represent the minimum age of first harvest (A), or the range of ages harvested (C). Curve B represents no minimum age limit (Reed 1980).

Uusi-Heikilla and others (2015) recently explored the effects of intensive harvest on Zebrafish populations to address the controversy of whether harvesting causes genetic as opposed to mere phenotypic changes.  The question is critically important to fisheries manager as genetic changes are more slowly reversible.  The use of Zebrafish allowed the investigators to follow populations over 5 generations of intense size selective pressure and examine functional genomic markers related to life history traits.   The parental stock began with 1500 wild-collected Zebrafish to ensure maximum genetic variation. The design employed three experimental treatments, with two replicates per treatment, and 450 zebrafish per replicate tank.  Harvest rate was 75% per generation.  Treatments were designed to mimic random size selection, minimum size limit (size selective), and maximum size limit (large size selection).  In addition to life history traits, the investigators examined 371 single nucleotide polymorphisms to see if allelic frequencies differed among treatments.

As hypothesized, the minimum length limits had surprising undesired genetic effects. Twenty-two loci showed genetic differentiation and eight of these were statistically significant.   After just five generations of harvesting, adult body size of the Zebrafish shrunk by 7%, which also affected the egg production of the surviving fish. The now-smaller Zebrafish produced fewer and smaller eggs!   Furthermore, the large-selected Zebrafish were significantly more explorative and bolder. This experiment supports a cause-and-effect relationship between size-selective harvest practices and changes in fish life history traits and productivity.  The implications for recovery of overfished populations are important.    

There are at least three options to restore overfished fisheries: (1) moratorium on all harvest; (2) daily creel limits or restrictive size limits, or (3) protect large fecund individuals via a maximum size limit or protected area.  A harvest moratorium rebuilt populations and restored age structure in Atlantic striped bass (Morone saxatilis) (Richards and Rago 1999; Secor, 2000). A transition from minimum length limit to a protected slot limit transformed an extremely truncated size distribution in a smallmouth bass fishery to a trophy fishery (Copeland et al. 2006).  The designation of marine protected reserves is a tool for protecting species from fishing (Bohnsack et al. 2004).   Reserves permit recovery of overexploited fish populations if larval export seeds exploited habitats (Harrison et al. 2012).  

Many commercially or recreationally important fish species have high fecundity and display a weak response between parents and recruits.  Big yearclasses can still be produced by low spawning stocks.  This leaves most fish managers with doubt about strong effects of size selective harvest.  Further, not all fish have the same life history and role. Small fish species should be exploited at levels well below those producing a maximum sustainable yield.  This will provide some forage fish to survive to feed higher trophic levels  (Pikitch et al 2014).  Fishing forage fish intensely increases likelihood of collapse (Essington et al. 2015).

The Atlantic menhaden is exploited with a reduction fishery (i.e, it reduces the catch, into fishmeal and fish oil) and a bait fishery. The fishery expanded from New England after the Civil War to the rest of the coastal states and peaked by 1950, when over 20 reduction factories processed the harvest.  Since the 1960s the menhaden fluctuated and reduction factories closed and reopened, until odor abatement regulations caused most of them to close.  Today a single reduction factory in Reedville, Virginia, processes the landings; Virginia is allocated 85% of the total allowable catch.  Menhaden constitute the largest landings, by volume, along the east coast, and rank second in the U.S behind the Alaskan Pollock. The Atlantic Menhaden were once overfished and sport and commercial fisheries dependent on the Atlantic Menhaden (e.g., Menhaden Defenders) have lobbied hard to change the way this fishery is managed.  Not too many forage fish have their own lobbying group.   Yet, the total allowable catch in recent years is still managed based on single species concepts, as if the Atlantic Menhaden was not eaten by a myriad of fish, mammals, and birds.  Talk about your inappropriate spherical cow!  

Based on the latest stock assessment, the Atlantic menhaden “stock status is not overfished and overfishing is not occurring” (SEDAR 2015).   But nothing in this 643-page assessment addresses the influence of abundance of Atlantic Menhaden on striped bass, bluefish, weakfish, tuna, or other coastal fishes.  Atlantic Menhaden holds the distinction of being the only fish species controlled by the General Assembly and not the Virginia Marine Resources Commission.    The politicians, not the professionals, make decisions. 

The Red Snapper fishery in the Gulf of Mexico collapsed in the late 1980s.  Because the fish and fishery occur in all Gulf states and federal waters, the fishery is managed by the Gulf Fishery Management Council under the authority of the Magnuson-Stevens Fisheries Management Act. The council was slow to enact protections and, therefore, the Red Snapper remained overfished for several decades.  Even in an overfished state, the economic value of the fisheries is $80 million.  The Red Snapper story illustrates why the simple theory was difficult to implement (Cowan et al. 2010).  First, bycatch of juvenile Red Snapper in shrimp trawls was not regulated and caused high mortality among immature Red Snapper.  Second, management plans required the allocation of allowable catch to recreational and commercial fisheries.  Third, regulations such as minimum size limits resulted in regulatory discard mortality, that is fish that are required by law to be released due to size, season, or bag requirements.  Undersized fish had to be released, much to the distress of anglers who assumed that many of the fish, caught from deeper waters and exhibiting protruding swimbladders, would die anyway.  Finally, artificial reefs and oil and gas platforms were deployed in the 1970s and 1980s and used as a red herring, as if the artificial structures would increase Red Snapper abundance (Galloway et al. 2009).   

The Red Snapper is a long-lived species that can live over 50 years, but some will mature at age 3 or 4. The large old fish are highly fecund.  Red Snapper have been called “bet hedgers” as an evolutionary strategy – females produce millions of very small eggs over her lifetime, with an infinitesimally small chance of surviving to be an adult Red Snapper.

Concept Map depicting cause and effect relationships of big old fat fecund female fish (BOFFF; Hixon et al. 2013)

Intense exploitation in the Gulf fishery truncates the age distribution such that few fish over 5 years old survive.  Fish are gaining weight at a very high rate at this age range from 5 to 20 years.  Also the fecundity is much higher for these big old fat fecund female fish  (BOFFFF; Hixon et al. 2013).  Older red snapper also spawn more frequently and a 32-inch female produces 24 times as many eggs as a 16-inch female (Porch et al. 2013).  

Growth and age distribution of Red Snapper in the Gulf of Mexico (Cowan et al. 2010).

One major factor affecting recruitment of Red Snapper is periodic occurrence of hypoxia, which caused the loss of all red snapper and most of the invertebrates on and around the reefs in 2001 (Workman and Foster 2002).   Since this periodic recruitment failure is an expected occurrence, it makes sense to maintain a larger spawning stock biomass so that large year classes of Red Snapper may be produced in good years.

Texas State record Red Snapper (40 pounds) caught June 1, 2014. Source

Yet the Red Snapper population has not recovered.  It took a long time to implement bycatch reduction in the shrimp trawlers.  Then many years later gear restrictions were implemented to limit hooking mortality and require gas bladder venting.  Yet only 72% of discarded Red Snapper survived after being released (Curtis et al. 2015).   Area closures in zones occupied by BOFFF Red Snapper have never been used; yet the closure of the area to fishing would reduce wasteful discards and discard mortality.  Yet the trend in fish biomass indicates only minor recent recovery.  The SEDAR Update (Gulf Fishery Management Council 2015) revealed that the stocks remain at historical lows and the age distribution is truncated.   Decisions made by the council have had a very high opportunity cost in terms of loss of fishery values over the past decades.

These three fishes teach us a lot about exploited populations of fish.  Any assessment of the status of an exploited fish population and evaluation of alternative management strategies requires certain assumptions.  However, sometimes I feel like choking on the spherical cow.   As we move forward we need to apply lessons learned from other fisheries and follow common sense (KISS) principles.

·      Little fish feed the world. Total allowable catch should be adjusted based on needs of predators or ecosystem services provided.

·      Protect the BOFFFF -- they survive unfavorable recruitment times and facilitate resilience.

·      Size-selective harvesting causes changes in key life-history traits, leading to low maximum body size and poor reproductive output.

·      Intense fishing selects for shy behaviors.

·      Effects of fishing proceed at a faster pace than the scientific approach to evaluating actions. The best available science often adopts simplifying assumptions.

References

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