The Seascapes

The Seascapes

Saturday, November 13, 2010

“Now where are all those kids I left by the side of the road?” I. Summer flounder life history & some observations of broad scale patterns.

Figure 1. Summer flounder range from Cape Canaveral,
 Florida to George's Bank off the Coast of Massachusetts. 
Subpopulations North and South of Cape Hattaras, North 
Carolina have different ecologies which are summarized 
in detail by Packer et al., 1999. Mapped data is available 
from Fishbase.
One of the things we have been trying to do is to link our discoveries in the adaptive plankton and bottom surveys of the seascapes with our understanding of the dynamics of fish and invertebrates important in the Mid-Atlantic Bight Ecosystem. The birth and death rates that underlie population dynamics occur to individuals with specific traits in habitats defined by sets of environmental factors affecting those rates (e.g. growth is dependent on temperatures; predation mortality is affected by the availability of hidey holes as well as other things such as body size). I am sure this is blindingly obvious to anyone who is not a marine biologist. But often we seem to ignore this because making the connection between local habitat effects on individuals and the emergent dynamics of populations at regional scales in a quantitative way is so hard. Somehow “information” is translated across scales and levels of biological organization in both directions, from local to regional scales (e.g. effects of individual births and deaths related to habitat quality within subpopulations), and from regional to local scales (e.g. connectivity among subpopulations provided by dispersing larvae and migrating adults that can supplement, rescue, or even found new subpopulations). Sometimes science is thinking out loud, and I am going try to amble through some data on summer flounder (AKA “fluke”) to try to understand these connections better.

Figure 2. Summer flounder life cycle in the mid-Atlantic Bight.
Adults spawn as they migrate in the autumn from nearshore 
coastal feeding grounds to deep overwintering habitats on 
the edge of the continental shelf. Fertilized eggs hatch larvae 
that live in the water column in the coastal ocean for a month
 or more before moving into estuaries and transforming into 
small juveniles. The juveniles overwinter in the estuaries.
Why choose summer flounder or “fluke” as a model species? One reason is that fluke integrate a particularly broad spectrum of habitats throughout the mid-Atlantic Bight (Fig. 2). During their complex life history which includes eggs and larvae that live in the water column as well as adults and juveniles strongly associated with the seabed, they use habitats ranging from marsh creeks in estuaries, to pelagic & seabed habitats in the coastal ocean, to deep habitats on the seabed at the edge of the continental shelf. From late spring through the summer, adults and juveniles are important predators in shallow estuaries and the near-shore coastal ocean. In early autumn they begin to migrate from these nearshore feeding habitats to deep overwintering grounds near the edge of the continental shelf. Adults two or more years old spawn as they migrate across the inner continental shelf. Each ripe female carries between ½ million to over 4 million eggs depending on her body size. The females release these eggs directly into the sea where they are externally fertilized by males. The fertilized eggs hatch in less than about 4 days. The larvae which are shaped like a regular fish (fusiform or laterally compressed) develop in the water column for 1 to 3 months depending on the temperature, availability of food, and their "ability" to avoid predators. Late stage larvae then move into estuaries during the winter. As they do, they metamorphose into small juvenile flatfish. Metamorphosis involves the “migration” of their eyes to the left side of their heads, and the twisting of their spinal cord into loop as they flatten into a flatfish. They are 10-20 millimeters long when they “settle” out of the water column to become more strongly associated with the bottom habitats in estuaries.

With the exception of fishing, we believe most of the important processes affecting birth and death rates occur from spawning through the early juvenile phase. Unfortunately for field ecologists working in the northern part of summer flounders range, the interesting habitat specific processes influencing summer flounder population dynamics happen during the autumn, winter, and spring when it's cold. Who named this fish? Why would a fish spawn in the autumn and float around in the plankton to settle in estuaries in the winter when its so cold? Our guess is that larvae and juveniles are probably exposed to fewer predators in the ocean and estuaries during the fall and winter than they would during the spring and summer. By growing slowly over the winter they achieve a size advantage when spring rolls around that makes them less vulnerable to predators but capable of eating larvae and early juveniles of other animals colonizing estuaries later in the spring. This is the expression of an important rarely violated ecological theorem: “You can't eat anything bigger than your head”.  However, the tradeoff for overwintering in the coastal ocean and estuaries for this subtropical flatfish is that it exposes larvae and small juveniles to cold winter temperatures in the northern part of the species range. Small juveniles die when exposed to temperatures below about 2 or 3 degrees Celsius in the laboratory (Keefe & Able 1993, Szedlemeyer et al., Malloy & Target, 1991, 1994 ).

Figure 3. Estimated trends in summer flounder population
size and age class diversity in the mid-Atlantic Bight. 
The population size was low and dominated by a few 
young age classes in the late 1980s early 1990 when a 
Fisheries Management Plan (FMP) was implemented. 
This plan was amended (Amend 2) in the early 1990s. In the
1990s and 2000s the population increased in size and older
fish became more abundant increasing the age class diversity. 

Another reason “fluke” are an interesting fish is that the mid-Atlantic bight population has been getting healthier over the past two decades. The estimated size of the population decreased from a peak in the early 1980s to a low about 1990 (Fig.3). During this time it was dominated by young fish and had a low age diversity. In response to this decline a fisheries management plan was established in late 1980s and amended in 1990s to restrict the amount of summer flounder that could be caught and the gear used to catch them. In the early 1990s the population began to rebound and older larger individuals became more common. The increased survival of older age classes is particularly important because higher age class diversity makes populations more resilient to environmental shocks. In part this is just simple bet hedging. But there is also evidence that older females produce more, higher quality eggs over a longer spawning periods than younger fish.  They are thought to produce kids more likely to survive the dangers of early life. This is the BOFFF hypothesis (Big, Old, Fat, Fecund, Females) . The summer flounder population is doing well enough that the current precautionary fishing regulations have become pretty contentious.

Figure 4. Estimated centers of summer flounder biomass 
during the autumn shifted toward the north east nearly
250 kilometers between 1973 and 2006. Centers of biomass
were calculated by fitting sample coordinates to year in a
generalized additive model that weighted the observations
by the standardized biomass of summer flounder collected
in North East Fisheries Center Bottom trawl surveys.  
But was the rebound of the summer flounder population entirely due to fisheries management and the reduction in predation by human predators? As the population increased the distribution of the animals also changed. The center of distribution of summer flounder in the Autumn seems to have shifted from an area offshore of Cape May, New Jersey in the 1970s, to one off the eastern end of Long Island, New York by the mid-2000s (Fig. 4). The velocity of this shift  was approximately 11 km per year from 1991 to 2005 (also see Janet Nye's work). These patterns suggest that changes in climate along with fishing may have affected the summer flounder population. Nineteen ninety one has been identified as the year in which the ecological dynamics of a number of marine populations changed. The summer flounder species range shift could have occurred because warmer temperatures allowed summer flounder to remain inshore on northern feeding grounds later in the autumn. However such a change in the timing of fall migration wouldn't necessarily result in an increase in population size. Alternatively, more early juvenile summer flounder may have survived more winters in northern estuarine nurseries because winter temperatures have been warmer in recent years. If this is the case summer flounder may be extending their species range to the north by coupling their life cycle and affecting affect the entire suite of habitats/ecosystems they use during their life history (Fig. 2). There is some evidence that the survival of young juveniles has increased in northern estuaries. Indices of Age-0 summer flounder abundance are have been higher in Massachusetts, Rhode Island and Connecticut (Fig. 5) than they have been in the past.  Furthermore winters have been warmer with fewer subfreezing degree days since the early 1990s (Fig. 5).  This is interesting since the survival of winter flounder juveniles on mid-Atlantic Bight estuarine nursery grounds has been poor since the 1990s apparently as a result of the high frequency of warm springs (Manderson, 2008).  Unlike summer flounder which are subtropical, winter flounder are a cold temperate flatfish.   

Figure 5. Standardized trends in age-0 summer flounder abundance estimated in surveys conducted in states ranging from Massachusetts (MA) to North Carolina (NC).  Abundance in Massachusetts, Rhode Island (RI), and Connecticut (CT) has been relatively high during the last decade (data from 47th SAW). This may have resulted from higher overwintering survival. Winters have been mild with fewer subfreezing degree days in the mid-Atlantic regions since the late 1980s based on analysis of daily temperature records compiled by the Academy of Natural Sciences in Philadelphia. New Jersey (NJ), Delaware (DE), Maryland(MD), Virginia (VA).

The effects of fishing regulations, and climate on migratory patterns and/or the survival of early juveniles summer flounder are alternative hypothesis of mechanisms that have could have different impacts on the mid-Atlantic ecosystem and implications for ecosystem and fisheries management. They are not necessary mutually exclusive, but may effect the population dynamics of summer flounder and the other animals they interact with simultaneously.   How can we find a method to estimate the relative contribution of fishing and changing climate on the habitat specific processes affecting the summer flounder population in the Mid-Atlantic Bight?

Keefe, M & KW Able, (1993) Patterns of metamorphosis in summer flounder, Paralichthys dentatus. Journal of Fish Biology 42(5): 1095-8649

Keefe M. & KW. Able (1994) Contributions of Abiotic and Biotic Factors to Settlement in Summer Flounder, Paralichthys dentatus. Copeia 1994 (2) 458-465 

Malloy, K. D., and T. E. Targett (1991) Feeding, growth and survival of juvenile summer flounder Paralichthys dentatus: experimental analysis of the effects of temperature and salinity. Mar. Ecol. Prog. Ser. 72:213-223.

Malloy, KD, and TE Targett. (1994) Effects of ration limitation and low temperature on growth, biochemical condition, and survival of juvenile summer flounder from two Atlantic Coast nurseries.
Trans. Am. Fish. Soc. 123:182–193.

Manderson. JP  (2008) The spatial scale of phase synchrony in winter flounder (Pseudopleuronectes americanus) production increased among southern New England nurseries in the 1990's. Canadian Journal of Fisheries and Aquatic Sciences 65:340-351.

Nye JA, Link JS, Hare JA, Overholtz WJ (2009) Changing spatial distribution of fish stocks in relation to climate and population size on the Northeast United States continental shelf. Mar Ecol Prog Ser 393:111-129

Szedlmayer, S. T., K. W. Able, and R. A. Roun-tree. 1992. Growth and ,temperature-induced mortality of young-of-the-year summer flounder (Paralichthys dentatus) in southern New Jersey. Copeia 1:120-128.

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