51813_Sturgeon biodioversity an.PDF

and increasingtotalmortality toZ= 0.23willreduce

theEPRvalue lo 20% ofEPRmax(Figure 2). For

paddlefish, increasing the total mortalityratefrom

Z= 0.30(current) toZ= 0.36willreduce theEPR

value to 50% ofEPRmaxand increasing therate toZ

= 0.45willreduce the value to 20% ofEPRmax(Fig-

ure 2).

As a group,chondrosteanpopulations aremore

sensitive to loss inreproductivepotentialcaused by

increases in the mortalityrate of age 7 and older fe-

malesthan arestripedbass,winterflounder, and

bluefishpopulations (Figure 3). The highersensi-

tivity of thechondrosteans tomortality in age 1 and

olderfish is due to acombination of characteristics

that determine their populationdynamics (Table 1).

Figure 3 .Effects ofincreasing the total mortalityrate ( Z ) ofage 1

and olderfemalesabove t h elevelwhenF= 0,on the correspond

The chondrosteans aregenerallylongerlived, are

later maturing and have lower natural mortality

fish. Thechondrosteans do notspawn everyyear

once they reachsexualmaturityand,except for At-

lantic sturgeon and whitesturgeon,havesubstan-

tiallylower fecunditythan theotherthreespecies I

examined. Alifehistorycharacteristicthat inte-

grates individualfecundity,naturalmortality, age at

maturity, and years betweensuccessive spawnings

is the age at which at least 50% of the maximum

lifetime egg production of an age 1 female is

(EPRmax).For white andAtlanticsturgeons, this age

is 3-10 times greater than the equivalent age for

stripedbass,winterflounder, andbluefish(Table 1 ) ;

Table 2.Reduction infishingmortalityrate (F)necessary to

ingpercentage ofthemaximuinlifetime egg production o fa n age

I female for whitesturgeon in theColumbiaRiverbelow the

BonnevilleDam. Atlanticsturgeon in theHudsonRivershort

Lake Ponchartrain, striped bass in the Hudson Rivcr, winter

flounder- in Cape cod Bay, and bluefish along the Atlantic coast.

therefore, theprobability ofsurvivingfrom age 1 to

the age of 50% ofmaximumlifetime egg production

isreduced by a power of3–10for the sturgeons.

rates than striped bass. winter flounder, and blue- nose sturgeon in the lower Connecticut River, paddlefish in

Restricting fishing mortality to offset losses from

achieved when no fishing mortality occurs other sources

For relatively long-lived species such as sturgeons

and paddlefish,a smallreduction infishingmortal-

ity on the age groupsvulnerable to harvest canoff-

set the effects of arelativelylarge reduction in age 0

survival.Thisrelationship ispossiblebecause the

age 0 fish are exposed to the risk of reduced survival

duringonly one year intheirlife;whereas,exposure

to the risk offishingspansmanyyears. As anexam-

ple.suppose the number of age 0 whitesturgeon in

the lowerColumbia River isreduced by 20% due to

contaminanttoxicity. A 20%reduction in age 0 sur-

0 0.370 0 vivalimpliesthat foreveryage 1 femalethat would

5 0.364 2 havesurvived her firstyearoflife, only0.8females

(^1520) 0.3450.352 (^57) value forpotentiallifetime eggproductionfrom 0.8
25 0.339 8 age^1 femaleswith thebaselinefishingmortality
30 0.331 11 rate ofF=0.37is equal tothe lifetime eggproduc-
tionofone age 1 female andafishingmortalityrate
achieveequivalentlifetime eggproduction of an age 1female
whitesturgeon in the ColumbiaRiverbelowBonneyille Dam
when thefraction offemalessurvivingfrom egg to age 1 (S 0 )is
reduced.
Reduction in F needed tomaintain Reduction in F
S 0 ( % ) equivalent lifetime egg (%)
production
10 0.358 (^3) are now survivingunder thealteredconditions.The