diff --git a/R/setFishing.R b/R/setFishing.R
index 1d1b70bb..933443e5 100644
--- a/R/setFishing.R
+++ b/R/setFishing.R
@@ -402,6 +402,25 @@ check_gear_params <- function(x) {
                 ". Did you perhaps mis-spell the names?")
     }
 
+    # Auto-populate missing argument columns for selectivity functions
+    if ("sel_func" %in% names(x)) {
+        funcs <- unique(x$sel_func)
+        funcs <- funcs[!is.na(funcs)]
+        for (sf in funcs) {
+            arg <- tryCatch({
+                args <- names(formals(sf))
+                args[!(args %in% c("w", "species_params", "..."))]
+            }, error = function(e) NULL)
+            if (length(arg) > 0) {
+                for (a in arg) {
+                    if (!(a %in% names(x))) {
+                        x[[a]] <- NA
+                    }
+                }
+            }
+        }
+    }
+
     # Validate parameters based on selectivity function
     if (nrow(x) > 0 && "sel_func" %in% names(x)) {
         for (i in seq_len(nrow(x))) {
diff --git a/tests/testthat/test-calc-selectivity.R b/tests/testthat/test-calc-selectivity.R
index 24d4a773..197bf2d0 100644
--- a/tests/testthat/test-calc-selectivity.R
+++ b/tests/testthat/test-calc-selectivity.R
@@ -37,7 +37,7 @@ test_that("calc_selectivity leaves unspecified gear-species combinations at zero
 
 test_that("calc_selectivity errors for missing or NA selectivity parameters", {
     params_missing <- knife_edge_selectivity_params_no_length
-
+    class(params_missing@gear_params) <- "data.frame"
     params_missing@gear_params$knife_edge_size <- NULL
     expect_error(calc_selectivity(params_missing),
                  "missing in the gear_params dataframe")
diff --git a/tests/testthat/test-setFishing.R b/tests/testthat/test-setFishing.R
index 3b4edc10..fd5cb1b8 100644
--- a/tests/testthat/test-setFishing.R
+++ b/tests/testthat/test-setFishing.R
@@ -415,3 +415,14 @@ test_that("gear_params reactive validation works", {
     df3 <- data.frame(species = "Sprat", gear = "g", sel_func = "knife_edge", knife_edge_size = -5)
     expect_warning(gp3 <- gear_params(df3), "knife_edge_size must be non-negative")
 })
+
+test_that("gear_params selectivity column auto-population works", {
+    df <- data.frame(species = "Sprat", gear = "g")
+    gp <- gear_params(df)
+    
+    # Setting sel_func to sigmoid_length should add columns l25 and l50
+    gp$sel_func <- "sigmoid_length"
+    expect_true(all(c("l25", "l50") %in% colnames(gp)))
+    expect_true(all(is.na(gp$l25)))
+    expect_true(all(is.na(gp$l50)))
+})